Add "external" prefix to methods and constructors.

lib/compiler/implementation/elements/elements.dart

Index: lib/compiler/implementation/elements/elements.dart
diff --git a/lib/compiler/implementation/elements/elements.dart b/lib/compiler/implementation/elements/elements.dart
index ad690789f5fbb8c414a1ab6995d0c5cfe0b77fdc..d05b3b67ec80855fc68e4edc861ae8af6c78b8b9 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -708,9 +708,14 @@ class FunctionElement extends Element {
 
   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;
   }