fix a few more codegen issues:

lib/src/codegen/js_codegen.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 208 matching lines @@
     var moduleDef = js.statement("dart_library.library(#, #, #, #, #)", [
       js.string(jsPath, "'"),
       _jsModuleValue ?? new JS.LiteralNull(),
       js.commentExpression(
           "Imports", new JS.ArrayInitializer(imports, multiline: true)),
       js.commentExpression("Lazy imports",
           new JS.ArrayInitializer(lazyImports, multiline: true)),
       module
     ]);
 
-    var jsBin = compiler.options.runnerOptions.v8Binary;
-
-    String scriptTag = null;
-    if (library.library.scriptTag != null) scriptTag = '/usr/bin/env $jsBin';
-    return new JS.Program(<JS.Statement>[moduleDef], scriptTag: scriptTag);
+    // TODO(jmesserly): scriptTag support.
+    // Enable this if we know we're targetting command line environment?
+    // It doesn't work in browser.
+    // var jsBin = compiler.options.runnerOptions.v8Binary;
+    // String scriptTag = null;
+    // if (library.library.scriptTag != null) scriptTag = '/usr/bin/env $jsBin';
+    return new JS.Program(<JS.Statement>[moduleDef]);
   }
 
   void _emitModuleItem(AstNode node) {
     // Attempt to group adjacent fields/properties.
     if (node is! VariableDeclaration) _flushLazyFields(_moduleItems);
     if (node is! FunctionDeclaration) _flushLibraryProperties(_moduleItems);
 
     var code = _visit(node);
     if (code != null) _moduleItems.add(code);
   }
@@ skipping 2940 matching lines @@
       return _privateNames.putIfAbsent(
           name, () => _initSymbol(new JS.TemporaryId(name)) as JS.TemporaryId);
     }
 
     if (name == '[]') {
       name = 'get';
     } else if (name == '[]=') {
       name = 'set';
     } else if (name == '-' && unary) {
       name = 'unary-';
+    } else if (name == 'constructor' || name == 'prototype') {
+      // This uses an illegal (in Dart) character for a member, avoiding the
+      // conflict. We could use practically any character for this.
+      name = '+$name';
     }
 
     // Dart "extension" methods. Used for JS Array, Boolean, Number, String.
     if (allowExtensions &&
         _extensionTypes.contains(type.element) &&
         !_objectMembers.containsKey(name)) {
       return js.call('dartx.#', _propertyName(name));
     }
 
     return _propertyName(name);
@@ skipping 107 matching lines @@
 
 /// A special kind of element created by the compiler, signifying a temporary
 /// variable. These objects use instance equality, and should be shared
 /// everywhere in the tree where they are treated as the same variable.
 class TemporaryVariableElement extends LocalVariableElementImpl {
   TemporaryVariableElement.forNode(Identifier name) : super.forNode(name);
 
   int get hashCode => identityHashCode(this);
   bool operator ==(Object other) => identical(this, other);
 }