[es6] Fix bug in pattern re-writing

As originally implemented, a SingleNameBinding within a BindingPattern
was incorrectly interpreted as an assignment if an initializer was
present and that initializer was itself an AssignmentExpresion.
For example:

    let x;
    { let [x = y = 1] = []; }
    print(x); // expected: undefined, actual: 1

Extend the heuristic that detects the "context" of a destructuring
pattern to account for AssignmentExpressions within SingleNameBindings.

BUG=v8:4891
LOG=N
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/1859423002

Cr-Commit-Position: refs/heads/master@{#35334}

src/parsing/pattern-rewriter.cc

@@ -79,7 +79,11 @@ bool Parser::PatternRewriter::IsBindingContext(PatternContext c) const {
 Parser::PatternRewriter::PatternContext
 Parser::PatternRewriter::SetAssignmentContextIfNeeded(Expression* node) {
   PatternContext old_context = context();
-  if (node->IsAssignment() && node->AsAssignment()->op() == Token::ASSIGN) {
+  // AssignmentExpressions may occur in the Initializer position of a
+  // SingleNameBinding. Such expressions should not prompt a change in the
+  // pattern's context.
+  if (node->IsAssignment() && node->AsAssignment()->op() == Token::ASSIGN &&
+      !IsInitializerContext()) {
     set_context(ASSIGNMENT);
   }
   return old_context;

test/mjsunit/es6/destructuring.js

@@ -260,6 +260,63 @@
 }());
 
 
+(function TestAssignmentExprInInitializers() {
+  {
+    let x, y;
+    {
+      let { x = y = 1 } = {};
+      assertSame(x, 1);
+      assertSame(y, 1);
+    }
+    assertSame(undefined, x);
+    assertSame(1, y);
+  }
+
+  {
+    let x, y;
+    {
+      let { x: x = y = 1 } = {};
+      assertSame(1, x);
+      assertSame(1, y);
+    }
+    assertSame(undefined, x);
+    assertSame(1, y);
+  }
+
+  {
+    let x, y;
+    {
+      let [ x = y = 1 ] = [];
+      assertSame(1, x);
+      assertSame(1, y);
+    }
+    assertSame(undefined, x);
+    assertSame(1, y);
+  }
+
+  {
+    let x, y;
+    (function({ x = y = 1 }) {}({}));
+    assertSame(undefined, x);
+    assertSame(1, y);
+  }
+
+  {
+    let x, y;
+    (function({ x: x = y = 1 }) {}({}));
+    assertSame(undefined, x);
+    assertSame(1, y);
+  }
+
+  {
+    let x, y;
+    (function([ x = y = 1 ]) {}([]));
+    assertSame(undefined, x);
+    assertSame(1, y);
+  }
+}());
+
+
 (function TestMultipleAccesses() {
   assertThrows(
     "'use strict';"+