dart2js: extract a native-blob handler and make it emitter-independent.

pkg/compiler/lib/src/js_emitter/native_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/native_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/native_emitter.dart b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
index b53c911a279667f15c954da266b59b361c3e8657..54d58e1fffd9feaf74a003611c7c3b4a27787b68 100644
--- a/pkg/compiler/lib/src/js_emitter/native_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/native_emitter.dart
@@ -422,4 +422,96 @@ class NativeEmitter {
 
     targetOutput.add('\n');
   }
+
+  /// Returns a JavaScript template that fills the embedded globals referenced
+  /// by [interceptorsByTagAccess] and [leafTagsAccess].
+  ///
+  /// This code must be invoked for every class that has a native info before
+  /// the program starts.
+  ///
+  /// The [infoAccess] parameter must evaluate to an expression that contains
+  /// the info (as a JavaScript string).
+  ///
+  /// The [constructorAccess] parameter must evaluate to an expression that
+  /// contains the constructor of the class. The constructor's prototype must
+  /// be set up.
+  ///
+  /// The [subclassReadGenerator] function must evaluate to a JS expression
+  /// that returns a reference to the constructor (with evaluated prototype)
+  /// of the given JS expression.
+  ///
+  /// The [interceptorsByTagAccess] must point to the embedded global
+  /// [embeddedNames.INTERCEPTORS_BY_TAG] and must be initialized with an empty
+  /// JS Object (used as a map).
+  ///
+  /// Similarly, the [leafTagsAccess] must point to the embedded global
+  /// [embeddedNames.LEAF_TAGS] and must be initialized with an empty JS Object
+  /// (used as a map).
+  ///
+  /// Both variables are passed in (instead of creating the access here) to
+  /// make sure the caller is aware of these globals.
+  jsAst.Statement buildNativeInfoHandler(
+      jsAst.Expression infoAccess,
+      jsAst.Expression constructorAccess,
+      jsAst.Expression subclassReadGenerator(jsAst.Expression subclass),
+      jsAst.Expression interceptorsByTagAccess,
+      jsAst.Expression leafTagsAccess) {
+    jsAst.Expression subclassRead =
+        subclassReadGenerator(js('subclasses[i]', []));
+    return js.statement('''
+          // The native info looks like this:
+          //
+          // HtmlElement: {
+          //     "%": "HTMLDivElement|HTMLAnchorElement;HTMLElement;FancyButton"
+          //
+          // The first two semicolon-separated parts contain dispatch tags, the
+          // third contains the JavaScript names for classes.
+          //
+          // The tags indicate that JavaScript objects with the dispatch tags
+          // (usually constructor names) HTMLDivElement, HTMLAnchorElement and
+          // HTMLElement all map to the Dart native class named HtmlElement.
+          // The first set is for effective leaf nodes in the hierarchy, the
+          // second set is non-leaf nodes.
+          //
+          // The third part contains the JavaScript names of Dart classes that
+          // extend the native class. Here, FancyButton extends HtmlElement, so
+          // the runtime needs to know that window.HTMLElement.prototype is the
+          // prototype that needs to be extended in creating the custom element.
+          //
+          // The information is used to build tables referenced by
+          // getNativeInterceptor and custom element support.
+          {
+            var nativeSpec = #info.split(";");
+            if (nativeSpec[0]) {
+              var tags = nativeSpec[0].split("|");
+              for (var i = 0; i < tags.length; i++) {
+                #interceptorsByTagAccess[tags[i]] = #constructor;
+                #leafTagsAccess[tags[i]] = true;
+              }
+            }
+            if (nativeSpec[1]) {
+              tags = nativeSpec[1].split("|");
+              if (#allowNativesSubclassing) {
+                if (nativeSpec[2]) {
+                  var subclasses = nativeSpec[2].split("|");
+                  for (var i = 0; i < subclasses.length; i++) {
+                    var subclass = #subclassRead;
+                    subclass.#nativeSuperclassTagName = tags[0];
+                  }
+                }
+                for (i = 0; i < tags.length; i++) {
+                  #interceptorsByTagAccess[tags[i]] = #constructor;
+                  #leafTagsAccess[tags[i]] = false;
+                }
+              }
+            }
+          }
+    ''', {'info': infoAccess,
+          'constructor': constructorAccess,
+          'subclassRead': subclassRead,
+          'interceptorsByTagAccess': interceptorsByTagAccess,
+          'leafTagsAccess': leafTagsAccess,
+          'nativeSuperclassTagName': embeddedNames.NATIVE_SUPERCLASS_TAG_NAME,
+          'allowNativesSubclassing': true});
+  }
 }