workaround for arrow function bind this

I filed dart-archive/dev_compiler#43 to track removal of this workaround

[email protected]

Review URL: https://chromereviews.googleplex.com/154077013

Author: John Messerly

pkg/dev_compiler/lib/src/codegen/js_codegen.dart

@@ -642,12 +642,15 @@ $name.prototype[Symbol.iterator] = function() {
       out.write(') ');
       node.body.accept(this);
     } else {
+      var bindThis = _needsBindThis(node.body);
+      if (bindThis) out.write("(");
       out.write("(");
       _visitNode(node.parameters);
       out.write(") => ");
       var body = node.body;
       if (body is ExpressionFunctionBody) body = body.expression;
       body.accept(this);
+      if (bindThis) out.write(").bind(this)");
     }
   }
 
@@ -674,11 +677,8 @@ $name.prototype[Symbol.iterator] = function() {
     if (e.enclosingElement is CompilationUnitElement &&
         (e.library != libraryInfo.library || _needsModuleGetter(e))) {
       out.write('${_jsLibraryName(e.library)}.');
-    } else if (currentClass != null) {
-      if (e is PropertyAccessorElement && !e.variable.isStatic ||
-          e is ClassMemberElement && !e.isStatic && e is! ConstructorElement) {
-        out.write('this.');
-      }
+    } else if (currentClass != null && _needsImplicitThis(e)) {
+      out.write('this.');
     }
     out.write(node.name);
   }
@@ -1244,7 +1244,9 @@ $name.prototype[Symbol.iterator] = function() {
       if (node.parent is! ExpressionStatement) {
         out.write('return ${_cascadeTarget.name};\n');
       }
-      out.write('})(', -2);
+      out.write('})', -2);
+      if (_needsBindThis(node.cascadeSections)) out.write('.bind(this)');
+      out.write('(');
       node.target.accept(this);
       out.write(')');
     }
@@ -1850,6 +1852,18 @@ $name.prototype[Symbol.iterator] = function() {
     name = _canonicalMethodName(name);
     return name.startsWith('[') ? name : '.$name';
   }
+
+  bool _needsBindThis(node) {
+    if (currentClass == null) return false;
+    var visitor = _BindThisVisitor._instance;
+    visitor._bindThis = false;
+    node.accept(visitor);
+    return visitor._bindThis;
+  }
+
+  static bool _needsImplicitThis(Element e) =>
+      e is PropertyAccessorElement && !e.variable.isStatic ||
+          e is ClassMemberElement && !e.isStatic && e is! ConstructorElement;
 }
 
 /// Returns true if the local variable is potentially mutated within [context].
@@ -1901,6 +1915,25 @@ class _AssignmentFinder extends RecursiveAstVisitor {
   }
 }
 
+/// This is a workaround for V8 arrow function bindings being not yet
+/// implemented. See issue #43
+class _BindThisVisitor extends RecursiveAstVisitor {
+  static _BindThisVisitor _instance = new _BindThisVisitor();
+  bool _bindThis = false;
+
+  @override
+  visitSimpleIdentifier(SimpleIdentifier node) {
+    if (JSCodegenVisitor._needsImplicitThis(node.staticElement)) {
+      _bindThis = true;
+    }
+  }
+
+  @override
+  visitThisExpression(ThisExpression node) {
+    _bindThis = true;
+  }
+}
+
 class JSGenerator extends CodeGenerator {
   JSGenerator(String outDir, Uri root, TypeRules rules)
       : super(outDir, root, rules);

pkg/dev_compiler/test/codegen/expect/BenchmarkBase/BenchmarkBase.js

@@ -53,12 +53,12 @@ var BenchmarkBase;
     }
     measure() {
       this.setup();
-      measureFor(() => {
+      measureFor((() => {
         this.warmup();
-      }, 100);
-      let result = measureFor(() => {
+      }).bind(this), 100);
+      let result = measureFor((() => {
         this.exercise();
-      }, 2000);
+      }).bind(this), 2000);
       this.teardown();
       return result;
     }

pkg/dev_compiler/test/codegen/expect/_internal/_internal.js

@@ -175,7 +175,7 @@ var _internal;
           result = ((_) => {
             _.length = this.length;
             return _;
-          })(new core.List());
+          }).bind(this)(new core.List());
         } else {
           result = new core.List(this.length);
         }
@@ -264,7 +264,7 @@ var _internal;
         let result = growable ? (((_) => {
           _.length = length;
           return _;
-        })(new core.List())) : new core.List(length);
+        }).bind(this)(new core.List())) : new core.List(length);
         for (let i = 0; i < length; i++) {
           result.set(i, this._iterable.elementAt(start + i));
           if (this._iterable.length < end) throw new core.ConcurrentModificationError(this);