Fix type error in js_dart2js.dart caught by DDC.

pkg/dev_compiler/lib/src/compiler/code_generator.dart

Index: pkg/dev_compiler/lib/src/compiler/code_generator.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/code_generator.dart b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
index d4069072afdc15a04bf9e717848d7a34c0434d1a..7a9560f1402218cd40026e1ac3b5c82d8d34926a 100644
--- a/pkg/dev_compiler/lib/src/compiler/code_generator.dart
+++ b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
@@ -1505,7 +1505,7 @@ class CodeGenerator extends GeneralizingAstVisitor
 
     var fnBody = js.call('this.noSuchMethod(new #.InvocationImpl(#, #, #))', [
       _runtimeModule,
-      _declareMemberName(method),
+      _declareMemberName(method, useDisplayName: true),
       positionalArgs,
       new JS.ObjectInitializer(invocationProps)
     ]);
@@ -1939,14 +1939,14 @@ class CodeGenerator extends GeneralizingAstVisitor
   }
 
   List<ExecutableElement> _extensionsToImplement(ClassElement element) {
-    var members = <ExecutableElement>[];
-    if (_extensionTypes.isNativeClass(element)) return members;
+    if (_extensionTypes.isNativeClass(element)) return [];
 
     // Collect all extension types we implement.
     var type = element.type;
     var types = _extensionTypes.collectNativeInterfaces(element);
-    if (types.isEmpty) return members;
+    if (types.isEmpty) return [];
 
+    var members = new Set<ExecutableElement>();
     // Collect all possible extension method names.
     var extensionMembers = new HashSet<String>();
     for (var t in types) {
@@ -1961,7 +1961,9 @@ class CodeGenerator extends GeneralizingAstVisitor
         members.add(m);
       }
     }
-    return members;
+    members.addAll(_collectMockMethods(type)
+        .where((m) => extensionMembers.contains(m.name)));
+    return members.toList();
   }
 
   /// Generates the implicit default constructor for class C of the form
@@ -5463,17 +5465,14 @@ class CodeGenerator extends GeneralizingAstVisitor
   ///
   /// Unlike call sites, we always have an element available, so we can use it
   /// directly rather than computing the relevant options for [_emitMemberName].
-  JS.Expression _declareMemberName(ExecutableElement e, {bool useExtension}) {
-    String name;
-    if (e is PropertyAccessorElement) {
-      name = e.variable.name;
-    } else {
-      name = e.name;
-    }
+  JS.Expression _declareMemberName(ExecutableElement e,
+      {bool useExtension, useDisplayName = false}) {
+    var name = (e is PropertyAccessorElement) ? e.variable.name : e.name;
     return _emitMemberName(name,
         isStatic: e.isStatic,
         useExtension:
-            useExtension ?? _extensionTypes.isNativeClass(e.enclosingElement));
+            useExtension ?? _extensionTypes.isNativeClass(e.enclosingElement),
+        useDisplayName: useDisplayName);
   }
 
   /// This handles member renaming for private names and operators.
@@ -5520,6 +5519,7 @@ class CodeGenerator extends GeneralizingAstVisitor
       {DartType type,
       bool isStatic: false,
       bool useExtension,
+      bool useDisplayName: false,
       Element element}) {
     // Static members skip the rename steps and may require JS interop renames.
     if (isStatic) {
@@ -5532,20 +5532,22 @@ class CodeGenerator extends GeneralizingAstVisitor
 
     // When generating synthetic names, we use _ as the prefix, since Dart names
     // won't have this (eliminated above), nor will static names reach here.
-    switch (name) {
-      case '[]':
-        name = '_get';
-        break;
-      case '[]=':
-        name = '_set';
-        break;
-      case 'unary-':
-        name = '_negate';
-        break;
-      case 'constructor':
-      case 'prototype':
-        name = '_$name';
-        break;
+    if (!useDisplayName) {
+      switch (name) {
+        case '[]':
+          name = '_get';
+          break;
+        case '[]=':
+          name = '_set';
+          break;
+        case 'unary-':
+          name = '_negate';
+          break;
+        case 'constructor':
+        case 'prototype':
+          name = '_$name';
+          break;
+      }
     }
 
     var result = _propertyName(name);