Support user generated custom native JS classes.

pkg/compiler/lib/src/patch_parser.dart

Index: pkg/compiler/lib/src/patch_parser.dart
diff --git a/pkg/compiler/lib/src/patch_parser.dart b/pkg/compiler/lib/src/patch_parser.dart
index 0f3008475c1095209ef5cf8d8f547788361d14dc..288b210de487291f30d4d47881c77c06114fab59 100644
--- a/pkg/compiler/lib/src/patch_parser.dart
+++ b/pkg/compiler/lib/src/patch_parser.dart
@@ -350,6 +350,7 @@ void patchClass(Compiler compiler,
   }
   origin.applyPatch(patch);
   checkNativeAnnotation(compiler, patch);
+  checkJsInteropAnnotation(compiler, patch);
 }
 
 /// Check whether [cls] has a `@Native(...)` annotation, and if so, set its
@@ -359,6 +360,12 @@ checkNativeAnnotation(Compiler compiler, ClassElement cls) {
       const NativeAnnotationHandler());
 }
 
+checkJsInteropAnnotation(Compiler compiler, ClassElement cls) {
+  EagerAnnotationHandler.checkAnnotation(compiler, cls,
+      const JsInteropAnnotationHandler());
+}
+
+
 /// Abstract interface for pre-resolution detection of metadata.
 ///
 /// The detection is handled in two steps:
@@ -449,6 +456,46 @@ class NativeAnnotationHandler implements EagerAnnotationHandler<String> {
   }
 }
 
+/// Annotation handler for pre-resolution detection of `@JsName(...)`
+/// annotations.
+class JsInteropAnnotationHandler implements EagerAnnotationHandler<String> {
+  const JsInteropAnnotationHandler();
+
+  String getJsNameAnnotation(MetadataAnnotation annotation) {
+    if (annotation.beginToken != null &&
+        annotation.beginToken.next.value == 'JsName') {
+      // Skipping '@', 'JsName', and '('.
+      Token argument = annotation.beginToken.next.next.next;
+      return (argument is StringToken) ? argument.value : '';
+    }
+
+    return null;
+  }
+
+  String apply(Compiler compiler,
+             Element element,
+             MetadataAnnotation annotation) {
+    if (element.isClass) {
+      String jsName = getJsNameAnnotation(annotation);
+      if (jsName != null) {
+        ClassElementX declaration = element.declaration;
+        // We do not track the class names for JsName annotations as we are not
+        // concerned about checked mode errors these classes.
+        declaration.isJsInterop = true;
+        return jsName;
+      }
+    }
+    return null;
+  }
+
+  @override
+  void validate(Compiler compiler,
+                Element element,
+                MetadataAnnotation annotation,
+                ConstantValue constant) {
+  }
+}
+
 /// Annotation handler for pre-resolution detection of `@patch` annotations.
 class PatchAnnotationHandler implements EagerAnnotationHandler<PatchVersion> {
   const PatchAnnotationHandler();