Allow patch files to add top-level declarations to the patched library.

lib/compiler/implementation/scanner/listener.dart

Index: lib/compiler/implementation/scanner/listener.dart
diff --git a/lib/compiler/implementation/scanner/listener.dart b/lib/compiler/implementation/scanner/listener.dart
index 7d18d4d8ed3ea36f354eb1bb7fb119a0ae8c23ac..0963033e335cdf0e755da3b1e5acce2ec2aa95fe 100644
--- a/lib/compiler/implementation/scanner/listener.dart
+++ b/lib/compiler/implementation/scanner/listener.dart
@@ -1514,6 +1514,17 @@ class PartialFunctionElement extends FunctionElement {
     if (patch !== null) return patch.position();
     return findMyName(beginToken);
   }
+
+  PartialFunctionElement cloneTo(Element enclosing,
+                                 DiagnosticListener listener) {
+    if (patch !== null) {
+      listener.cancel("Cloning a patched function.", element: this);
+    }
+    PartialFunctionElement result = new PartialFunctionElement(
+        name, beginToken, getOrSet, endToken, kind, modifiers, enclosing);
+    result.scriptOverride = getScript();
+    return result;
+  }
 }
 
 class PartialFieldListElement extends VariableListElement {
@@ -1543,6 +1554,14 @@ class PartialFieldListElement extends VariableListElement {
   }
 
   Token position() => beginToken; // findMyName doesn't work. I'm nameless.
+
+  PartialFieldListElement cloneTo(Element enclosing,
+                                  DiagnosticListener listener) {
+    PartialFieldListElement result = new PartialFieldListElement(
+        beginToken, endToken, modifiers, enclosing);
+    result.scriptOverride = getScript();
+    return result;
+  }
 }
 
 class PartialTypedefElement extends TypedefElement {
@@ -1560,6 +1579,14 @@ class PartialTypedefElement extends TypedefElement {
   }
 
   position() => findMyName(token);
+
+  PartialTypedefElement cloneTo(Element enclosing,
+                                DiagnosticListener listener) {
+    PartialTypedefElement result =
+        new PartialTypedefElement(name, enclosing, token);
+    result.scriptOverride = getScript();
+    return result;
+  }
 }
 
 Node parse(DiagnosticListener diagnosticListener,