Add "external" prefix to methods and constructors.

lib/compiler/implementation/elements/elements.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('elements');
 
 #import('../tree/tree.dart');
 #import('../scanner/scannerlib.dart');
 #import('../leg.dart');  // TODO(karlklose): we only need type.
 #import('../util/util.dart');
@@ skipping 690 matching lines @@
   }
 
   FunctionType computeType(Compiler compiler) {
     if (type != null) return type;
     type = compiler.computeFunctionType(this, computeSignature(compiler));
     return type;
   }
 
   Node parseNode(DiagnosticListener listener) {
     if (cachedNode !== null) return cachedNode;
-    if (patch !== null) {
-      cachedNode = patch.parseNode(listener);
+    if (patch === null) {
+      if (modifiers.isExternal()) {
+        // An external function that isn't patched has no body.
+        return null;
+      }
+      return cachedNode;
     }
+    cachedNode = patch.parseNode(listener);
     return cachedNode;
   }
 
   Token position() => cachedNode.getBeginToken();
 
   FunctionElement asFunctionElement() => this;
 }
 
 class ConstructorBodyElement extends FunctionElement {
   FunctionElement constructor;
@@ skipping 412 matching lines @@
   final Node node;
   Type bound;
   Type type;
   TypeVariableElement(name, Element enclosing, this.node, this.type,
                       [this.bound])
     : super(name, ElementKind.TYPE_VARIABLE, enclosing);
   Type computeType(compiler) => type;
   Node parseNode(compiler) => node;
   toString() => "${enclosingElement.toString()}.${name.slowToString()}";
 }