Move all the builtin$ interceptors to a new interceptors library and shorten the names of them.

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 734b911932bb3551eb33909957a76de027d69aca..be1ca59a293012d48214c78478ab4762fd1bd68f 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -301,10 +301,21 @@ class LibraryElement extends CompilationUnitElement {
     }
   }
 
+  /** Look up a top-level element in this library. The element could
+    * potentially have been imported from another library. Returns
+    * null if no such element exist. */
   Element find(SourceString elementName) {
     return elements[elementName];
   }
 
+  /** Look up a top-level element in this library, but only look for
+    * non-imported elements. Returns null if no such element exist. */
+  Element findLocal(SourceString elementName) {
+    Element result = elements[elementName];
+    if (result === null || result.getLibrary() != this) return null;
+    return result;
+  }
+
   void forEachExport(f(Element element)) {
     elements.forEach((SourceString _, Element e) {
       if (this === e.getLibrary()