Add tests for interpretation of `let` token

test/language/statements/for-in/head-let-destructuring.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-iteration-statements
+es6id: 13.7
+description: >
+  The token sequence `let [`is interpreted as the beginning of a destructuring
+  binding pattern
+info: |
+  Syntax
+
+  IterationStatement[Yield, Return]:
+
+    for ( [lookahead ∉ { let [ } ] LeftHandSideExpression[?Yield] in
+      Expression[+In, ?Yield] ) Statement[?Yield, ?Return]
+
+    for ( ForDeclaration[?Yield] in Expression[+In, ?Yield] )
+      Statement[?Yield, ?Return]
+---*/
+
+var obj = Object.create(null);
+var value;
+
+obj.key = 1;
+
+for ( let[x] in obj ) {
+  value = x;
+}
+
+assert.sameValue(typeof x, 'undefined', 'binding is block-scoped');
+assert.sameValue(value, 'k');

test/language/statements/for-in/head-lhs-let.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-iteration-statements
+es6id: 13.7
+description: >
+  The `let` token is interpreted as an Identifier when it is not followed by a
+  `[` token
+info: |
+  Syntax
+
+  IterationStatement[Yield, Return]:
+
+    for ( [lookahead ∉ { let [ } ] LeftHandSideExpression[?Yield] in
+      Expression[+In, ?Yield] ) Statement[?Yield, ?Return]
+
+    for ( ForDeclaration[?Yield] in Expression[+In, ?Yield] )
+      Statement[?Yield, ?Return]
+flags: [noStrict]
+---*/
+
+var obj = Object.create(null);
+var let, value;
+
+obj.key = 1;
+
+for ( let in obj ) ;
+
+assert.sameValue(let, 'key', 'IdentifierReference');
+
+Object.defineProperty(Array.prototype, '1', {
+  set: function(param) {
+    value = param;
+  }
+});
+for ( [let][1] in obj ) ;
+
+assert.sameValue(value, 'key', 'MemberExpression');

test/language/statements/for-of/head-let-destructuring.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-for-in-and-for-of-statements
+es6id: 13.7.5
+description: >
+  The token sequence `let [`is interpreted as the beginning of a destructuring
+  binding pattern
+info: |
+  Syntax
+
+  IterationStatement[Yield, Return]:
+
+    for ( [lookahead ≠ let]LeftHandSideExpression[?Yield] of
+      AssignmentExpression[+In, ?Yield] ) Statement[?Yield, ?Return]
+
+    for ( ForDeclaration[?Yield] of AssignmentExpression[+In, ?Yield] )
+      Statement[?Yield, ?Return]
+---*/
+
+var value;
+
+for ( let[x] of [[34]] ) {
+  value = x;
+}
+
+assert.sameValue(typeof x, 'undefined', 'binding is block-scoped');
+assert.sameValue(value, 34);

test/language/statements/for-of/head-lhs-let.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-for-in-and-for-of-statements
+es6id: 13.7.5
+description: >
+  The `let` token is disallowed when not followed by a `[` token
+info: |
+  Syntax
+
+  IterationStatement[Yield, Return]:
+
+    for ( [lookahead ≠ let]LeftHandSideExpression[?Yield] of
+      AssignmentExpression[+In, ?Yield] ) Statement[?Yield, ?Return]
+
+    for ( ForDeclaration[?Yield] of AssignmentExpression[+In, ?Yield] )
+      Statement[?Yield, ?Return]
+negative: SyntaxError
+---*/
+
+for ( let of [] ) ;

test/language/statements/for/head-let-destructuring.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-iteration-statements
+es6id: 13.7
+description: >
+  The token sequence `let [`is interpreted as the beginning of a destructuring
+  binding pattern
+info: |
+  Syntax
+
+  IterationStatement[Yield, Return]:
+
+    for ( [lookahead ∉ { let [ } ] Expression[~In, ?Yield]opt ;
+      Expression[+In, ?Yield]opt ; Expression[+In, ?Yield]opt )
+      Statement[?Yield, ?Return]
+
+    for ( LexicalDeclaration[~In, ?Yield] Expression[+In, ?Yield]opt ;
+      Expression[+In, ?Yield]opt) Statement[?Yield, ?Return]
+---*/
+
+var value;
+
+for ( let[x] = [23]; ; ) {
+  value = x;
+  break;
+}
+
+assert.sameValue(typeof x, 'undefined', 'binding is block-scoped');
+assert.sameValue(value, 23);

test/language/statements/for/head-lhs-let.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-iteration-statements
+es6id: 13.7
+description: >
+  The `let` token is interpreted as an Identifier when it is not followed by a
+  `[` token
+info: |
+  Syntax
+
+  IterationStatement[Yield, Return]:
+
+    for ( [lookahead ∉ { let [ } ] Expression[~In, ?Yield]opt ;
+      Expression[+In, ?Yield]opt ; Expression[+In, ?Yield]opt )
+      Statement[?Yield, ?Return]
+
+    for ( LexicalDeclaration[~In, ?Yield] Expression[+In, ?Yield]opt ;
+      Expression[+In, ?Yield]opt) Statement[?Yield, ?Return]
+flags: [noStrict]
+---*/
+
+var let;
+
+let = 1;
+for ( let; ; )
+  break;
+
+assert.sameValue(let, 1, 'IdentifierReference');
+
+let = 2;
+for ( let = 3; ; )
+  break;
+
+assert.sameValue(let, 3, 'AssignmentExpression');
+
+let = 4;
+for ( [let][0]; ; )
+  break;
+
+assert.sameValue(let, 4, 'MemberExpression');