Support user generated custom native JS classes.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 65742316c66ff25e990a162e3b8f69e1f1b68f9f..e0be992817bbf3f14c19ad2e136952472584839e 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -225,9 +225,30 @@ abstract class ElementX extends Element with ElementCommon {
 
   String _fixedBackendName = null;
   bool _isNative = false;
-  bool get isNative => _isNative;
-  bool get hasFixedBackendName => _fixedBackendName != null;
-  String get fixedBackendName => _fixedBackendName;
+  String _jsInteropName = null;
+  String get jsInteropName => _jsInteropName;
+
+  bool get isJsInterop => _jsInteropName != null;
+
+  void setJsInterop(String name) { _jsInteropName = name; }
+
+  bool get isNative => _isNative || isJsInterop;
+  bool get hasFixedBackendName => fixedBackendName != null || isJsInterop;
+
+  String _jsNameHelper(Element e) {
+    if (e._jsInteropName != null && e._jsInteropName.isNotEmpty)
+      return e._jsInteropName;
+    return e.isLibrary ? 'self' : e.name;
+  }
+
+  String get fixedBackendName {
+    if (_fixedBackendName != null) return _fixedBackendName;
+    if (isJsInterop) {
+      return _jsNameHelper(isConstructor ? enclosingClass : this);
+    }
+    return null;
+  }
+
   // Marks this element as a native element.
   void setNative(String name) {
     _isNative = true;
@@ -1802,6 +1823,20 @@ abstract class BaseFunctionElementX
     return functionSignatureCache;
   }
 
+  /**
+   * An function is part of JsInterop if it has a jsInteropName annotation or it
+   * is an external class or top level member of a class or library tagged as
+   * JsInterop.
+   */
+  bool get isJsInterop {
+    if (!isExternal) return false;
+
+    if (super.isJsInterop) return true;
+    if (isClassMember) return contextClass.isJsInterop;
+    if (isTopLevel) return library.isJsInterop;
+    return false;
+  }
+
   List<ParameterElement> get parameters {
     // TODO(johnniwinther): Store the list directly, possibly by using List
     // instead of Link in FunctionSignature.
@@ -2419,7 +2454,7 @@ abstract class BaseClassElementX extends ElementX
     return asInstanceOf(compiler.functionClass) != null || callType != null;
   }
 
-  bool get isNative => nativeTagInfo != null;
+  bool get isNative => nativeTagInfo != null || isJsInterop;
 
   void setNative(String name) {
     // TODO(johnniwinther): Assert that this is only called once. The memory