diff --git a/sdk/lib/_internal/compiler/implementation/library_loader.dart b/sdk/lib/_internal/compiler/implementation/library_loader.dart
index b7ceefc04d9..1217a113625 100644
--- a/sdk/lib/_internal/compiler/implementation/library_loader.dart
+++ b/sdk/lib/_internal/compiler/implementation/library_loader.dart
@@ -498,23 +498,33 @@ class _LibraryLoaderTask extends CompilerTask implements LibraryLoaderTask {
       return new Future.value(library);
     }
     return compiler.withCurrentElement(importingLibrary, () {
-      return compiler.readScript(node, readableUri)
-          .then((Script script) {
-            if (script == null) return null;
-            LibraryElement element = new LibraryElementX(script, resolvedUri);
-            compiler.withCurrentElement(element, () {
-              handler.registerNewLibrary(element);
-              native.maybeEnableNative(compiler, element);
-              libraryCanonicalUriMap[resolvedUri] = element;
-              compiler.scanner.scanLibrary(element);
-            });
-            return processLibraryTags(handler, element).then((_) {
-              compiler.withCurrentElement(element, () {
-                handler.registerLibraryExports(element);
-              });
-              return element;
-            });
+      return compiler.readScript(node, readableUri).then((Script script) {
+        if (script == null) return null;
+        LibraryElement element =
+            createLibrarySync(handler, script, resolvedUri);
+        return processLibraryTags(handler, element).then((_) {
+          compiler.withCurrentElement(element, () {
+            handler.registerLibraryExports(element);
           });
+          return element;
+        });
+      });
+    });
+  }
+
+  LibraryElement createLibrarySync(
+      LibraryDependencyHandler handler,
+      Script script,
+      Uri resolvedUri) {
+    LibraryElement element = new LibraryElementX(script, resolvedUri);
+    return compiler.withCurrentElement(element, () {
+      if (handler != null) {
+        handler.registerNewLibrary(element);
+        libraryCanonicalUriMap[resolvedUri] = element;
+      }
+      native.maybeEnableNative(compiler, element);
+      compiler.scanner.scanLibrary(element);
+      return element;
     });
   }
 }
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
index 234352bb34e..468473e83ce 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
@@ -2169,7 +2169,7 @@ class NodeListener extends ElementListener {
   }
 }
 
-abstract class PartialElement implements Element {
+abstract class PartialElement {
   Token get beginToken;
   Token get endToken;
 
@@ -2254,16 +2254,17 @@ class PartialConstructorElement extends ConstructorElementX
   }
 }
 
-class PartialFieldList extends VariableList {
+class PartialFieldList extends VariableList with PartialElement {
   final Token beginToken;
   final Token endToken;
-  final bool hasParseError;
 
   PartialFieldList(this.beginToken,
                    this.endToken,
                    Modifiers modifiers,
-                   this.hasParseError)
-      : super(modifiers);
+                   bool hasParseError)
+      : super(modifiers) {
+    super.hasParseError = hasParseError;
+  }
 
   VariableDefinitions parseNode(Element element, DiagnosticListener listener) {
     if (definitions != null) return definitions;
@@ -2365,7 +2366,8 @@ Node parse(DiagnosticListener diagnosticListener,
     doParse(new Parser(listener));
   } on ParserError catch (e) {
     if (element is PartialElement) {
-      element.hasParseError = true;
+      PartialElement partial = element as PartialElement;
+      partial.hasParseError = true;
     }
     return new ErrorNode(element.position, e.reason);
   }
diff --git a/site/try/poi/diff.dart b/site/try/poi/diff.dart
new file mode 100644
index 00000000000..a2e17eb4201
--- /dev/null
+++ b/site/try/poi/diff.dart
@@ -0,0 +1,134 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library trydart.poi.diff;
+
+import 'dart:async' show
+    Completer,
+    Future,
+    Stream;
+
+import 'dart:convert' show
+    LineSplitter,
+    UTF8;
+
+import 'package:compiler/compiler.dart' as api;
+
+import 'package:compiler/implementation/dart2jslib.dart' show
+    Compiler,
+    Enqueuer,
+    QueueFilter,
+    Script,
+    WorkItem;
+
+import 'package:compiler/implementation/elements/visitor.dart' show
+    ElementVisitor;
+
+import 'package:compiler/implementation/elements/elements.dart' show
+    AbstractFieldElement,
+    ClassElement,
+    CompilationUnitElement,
+    Element,
+    ElementCategory,
+    FunctionElement,
+    LibraryElement,
+    ScopeContainerElement;
+
+import 'package:compiler/implementation/elements/modelx.dart' as modelx;
+
+import 'package:compiler/implementation/dart_types.dart' show
+    DartType;
+
+import 'package:compiler/implementation/scanner/scannerlib.dart' show
+    EOF_TOKEN,
+    ErrorToken,
+    IDENTIFIER_TOKEN,
+    KEYWORD_TOKEN,
+    PartialClassElement,
+    PartialElement,
+    Token;
+
+import 'package:compiler/implementation/source_file.dart' show
+    StringSourceFile;
+
+class Difference {
+  final Element before;
+  final Element after;
+  Token token;
+
+  Difference(this.before, this.after);
+
+  String toString() {
+    if (before == null) return 'Added($after)';
+    if (after == null) return 'Removed($before)';
+    return 'Modified($after -> $before)';
+  }
+}
+
+List<Difference> computeDifference(
+    ScopeContainerElement before,
+    ScopeContainerElement after) {
+  Map<String, Element> beforeMap = <String, Element>{};
+  before.forEachLocalMember((Element element) {
+    beforeMap[element.name] = element;
+  });
+  List<Difference> modifications = <Difference>[];
+  List<Difference> potentiallyChanged = <Difference>[];
+  after.forEachLocalMember((Element element) {
+    Element existing = beforeMap.remove(element.name);
+    if (existing == null) {
+      modifications.add(new Difference(null, element));
+    } else {
+      potentiallyChanged.add(new Difference(existing, element));
+    }
+  });
+
+  modifications.addAll(
+      beforeMap.values.map((Element element) => new Difference(element, null)));
+
+  modifications.addAll(
+      potentiallyChanged.where(areDifferentElements));
+
+  return modifications;
+}
+
+bool areDifferentElements(Difference diff) {
+  Element beforeElement = diff.before;
+  Element afterElement = diff.after;
+  var before = (beforeElement is modelx.VariableElementX)
+      ? beforeElement.variables : beforeElement;
+  var after = (afterElement is modelx.VariableElementX)
+      ? afterElement.variables : afterElement;
+  if (before is PartialElement && after is PartialElement) {
+    Token beforeToken = before.beginToken;
+    Token afterToken = after.beginToken;
+    Token stop = before.endToken;
+    int beforeKind = beforeToken.kind;
+    int afterKind = afterToken.kind;
+    while (beforeKind != EOF_TOKEN && afterKind != EOF_TOKEN) {
+
+      if (beforeKind != afterKind) {
+        diff.token = afterToken;
+        return true;
+      }
+
+      if (beforeToken is! ErrorToken && afterToken is! ErrorToken) {
+        if (beforeToken.value != afterToken.value) {
+          diff.token = afterToken;
+          return true;
+        }
+      }
+
+      if (beforeToken == stop) return false;
+
+      beforeToken = beforeToken.next;
+      afterToken = afterToken.next;
+      beforeKind = beforeToken.kind;
+      afterKind = afterToken.kind;
+    }
+    return beforeKind != afterKind;
+  }
+  print("$before isn't a PartialElement");
+  return true;
+}
diff --git a/tests/try/poi/diff_test.dart b/tests/try/poi/diff_test.dart
new file mode 100644
index 00000000000..ae384cf80a4
--- /dev/null
+++ b/tests/try/poi/diff_test.dart
@@ -0,0 +1,123 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// Test of element diff.
+library trydart.diff_test;
+
+import 'dart:async' show
+    Future;
+
+import 'package:expect/expect.dart' show
+    Expect;
+
+import 'package:async_helper/async_helper.dart' show
+    asyncTest;
+
+import 'package:compiler/implementation/dart2jslib.dart' show
+    Compiler,
+    Script;
+
+import 'package:compiler/implementation/source_file.dart' show
+    StringSourceFile;
+
+import 'package:compiler/implementation/elements/elements.dart' show
+    Element,
+    LibraryElement;
+
+import 'package:try/poi/diff.dart' show
+    Difference,
+    computeDifference;
+
+import '../../compiler/dart2js/compiler_helper.dart' show
+    MockCompiler,
+    compilerFor;
+
+final TEST_DATA = [
+    {
+      'beforeSource': 'main() {}',
+      'afterSource': 'main() { var x; }',
+      'expectations': [['main', 'main']],
+    },
+    {
+      'beforeSource': 'main() {}',
+      'afterSource': 'main() { /* ignored */ }',
+      'expectations': [],
+    },
+    {
+      'beforeSource': 'main() {}',
+      'afterSource': 'main() { }',
+      'expectations': [],
+    },
+    {
+      'beforeSource': 'var i; main() {}',
+      'afterSource': 'main() { } var i;',
+      'expectations': [],
+    },
+    {
+      'beforeSource': 'main() {}',
+      'afterSource': '',
+      'expectations': [['main', null]],
+    },
+    {
+      'beforeSource': '',
+      'afterSource': 'main() {}',
+      'expectations': [[null, 'main']],
+    },
+];
+
+const String SCHEME = 'org.trydart.diff-test';
+
+Uri customUri(String path) => Uri.parse('$SCHEME://$path');
+
+Future<List<Difference>> testDifference(
+    String beforeSource,
+    String afterSource) {
+  Uri scriptUri = customUri('main.dart');
+  MockCompiler compiler = compilerFor(beforeSource, scriptUri);
+
+  Future<LibraryElement> future = compiler.libraryLoader.loadLibrary(scriptUri);
+  return future.then((LibraryElement library) {
+    Script sourceScript = new Script(
+        scriptUri, scriptUri, new StringSourceFile('$scriptUri', afterSource));
+    var dartPrivacyIsBroken = compiler.libraryLoader;
+    LibraryElement newLibrary = dartPrivacyIsBroken.createLibrarySync(
+        null, sourceScript, scriptUri);
+    return computeDifference(library, newLibrary);
+  });
+}
+
+Future testData(Map data) {
+  String beforeSource = data['beforeSource'];
+  String afterSource = data['afterSource'];
+  List expectations = data['expectations'];
+
+  validate(List<Difference> differences) {
+    return checkExpectations(expectations, differences);
+  }
+
+  return testDifference(beforeSource, afterSource).then(validate);
+}
+
+String elementNameOrNull(Element element) {
+  return element == null ? null : element.name;
+}
+
+checkExpectations(List expectations, List<Difference> differences) {
+  Iterator iterator = expectations.iterator;
+  for (Difference difference in differences) {
+    Expect.isTrue(iterator.moveNext());
+    List expectation = iterator.current;
+    String expectedBeforeName = expectation[0];
+    String expectedAfterName = expectation[1];
+    Expect.stringEquals(
+        expectedBeforeName, elementNameOrNull(difference.before));
+    Expect.stringEquals(expectedAfterName, elementNameOrNull(difference.after));
+    print(difference);
+  }
+  Expect.isFalse(iterator.moveNext());
+}
+
+void main() {
+  asyncTest(() => Future.forEach(TEST_DATA, testData));
+}