Support user generated custom native JS classes.

pkg/compiler/lib/src/js_backend/namer.dart

Index: pkg/compiler/lib/src/js_backend/namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index 6156b277c2107c48dd750947b0bb977e9ad7b05a..5f762056f3ad2a4755cd4f1433abf8a9378e4fa3 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -633,6 +633,26 @@ class Namer {
     return invocationName(new Selector.fromElement(method));
   }
 
+  String _jsNameHelper(Element e) {
+    if (e.jsInteropName != null && e.jsInteropName.isNotEmpty)
+      return e.jsInteropName;
+    return e.isLibrary ? 'self' : e.name;
+  }
+
+  String fixedBackendPath(Element element) {
+    if (!element.isJsInterop) return null;
+    if (element.isInstanceMember) return 'this';
+    if (element.isConstructor) return fixedBackendPath(element.enclosingClass);
+    if (element.isLibrary) return 'self';
+    var sb = new StringBuffer();
+    sb..write(_jsNameHelper(element.library));
+
+    if (element.enclosingClass != null && element.enclosingClass != element) {
+      sb..write('.')..write(_jsNameHelper(element.enclosingClass));
+    }
+    return sb.toString();
+  }
+
   /// Returns the annotated name for a variant of `call`.
   /// The result has the form:
   ///
@@ -764,6 +784,7 @@ class Namer {
     if (element.hasFixedBackendName) {
       // Certain native fields must be given a specific name. Native names must
       // not contain '$'. We rely on this to avoid clashes.
+      // TODO(jacobr): we need to relax this constraint.
       assert(enclosingClass.isNative &&
              !element.fixedBackendName.contains(r'$'));