Support serialization of the compiler backbone.

pkg/compiler/lib/src/elements/common.dart

Index: pkg/compiler/lib/src/elements/common.dart
diff --git a/pkg/compiler/lib/src/elements/common.dart b/pkg/compiler/lib/src/elements/common.dart
index 8c8c4ffb2085b2f01d3a44f7b7c70e91a9c2a8b5..f9013c609ff3ce620794df46253f7c35bcf7aa7b 100644
--- a/pkg/compiler/lib/src/elements/common.dart
+++ b/pkg/compiler/lib/src/elements/common.dart
@@ -126,6 +126,28 @@ abstract class LibraryElementCommon implements LibraryElement {
   @override
   bool get isInternalLibrary =>
       isPlatformLibrary && canonicalUri.path.startsWith('_');
+
+  String getLibraryOrScriptName() {
+    if (hasLibraryName()) {
+      return getLibraryName();
+    } else {
+      // Use the file name as script name.
+      String path = canonicalUri.path;
+      return path.substring(path.lastIndexOf('/') + 1);
+    }
+  }
+
+  int compareTo(LibraryElement other) {
+    if (this == other) return 0;
+    return getLibraryOrScriptName().compareTo(other.getLibraryOrScriptName());
+  }
+}
+
+abstract class CompilationUnitElementCommon implements CompilationUnitElement {
+  int compareTo(CompilationUnitElement other) {
+    if (this == other) return 0;
+    return '${script.readableUri}'.compareTo('${other.script.readableUri}');
+  }
 }
 
 abstract class ClassElementCommon implements ClassElement {