Reapply "Make patch file import declarations apply to the patched library too."

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 e2dcc2add44cf35966a395936cf764fb7a9585b6..ad690789f5fbb8c414a1ab6995d0c5cfe0b77fdc 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -386,11 +386,18 @@ class LibraryElement extends CompilationUnitElement {
 class PrefixElement extends Element {
   Map<SourceString, Element> imported;
   Token firstPosition;
+  final CompilationUnitElement patchSource;
 
-  PrefixElement(SourceString prefix, Element enclosing, this.firstPosition)
+  PrefixElement(SourceString prefix, Element enclosing, this.firstPosition,
+                [this.patchSource])
     : imported = new Map<SourceString, Element>(),
       super(prefix, ElementKind.PREFIX, enclosing);
 
+  CompilationUnitElement getCompilationUnit() {
+    if (patchSource !== null) return patchSource;
+    return super.getCompilationUnit();
+  }
+
   lookupLocalMember(SourceString memberName) => imported[memberName];
 
   Type computeType(Compiler compiler) => compiler.types.dynamicType;