Fix for resolving interface constructors in factory, issue 521.

compiler/java/com/google/dart/compiler/util/StringUtils.java

Index: compiler/java/com/google/dart/compiler/util/StringUtils.java
diff --git a/compiler/java/com/google/dart/compiler/util/StringUtils.java b/compiler/java/com/google/dart/compiler/util/StringUtils.java
new file mode 100644
index 0000000000000000000000000000000000000000..91a1a0f7bf39402364c3c5a2f62eb4f7fb610ff4
--- /dev/null
+++ b/compiler/java/com/google/dart/compiler/util/StringUtils.java
@@ -0,0 +1,36 @@
+// Copyright (c) 2011, 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.
+package com.google.dart.compiler.util;
+
+/**
+ * Utilities for {@link String}.
+ */
+public class StringUtils {
+  /**
+   * The empty String <code>""</code>.
+   */
+  public static final String EMPTY = "";
+
+  /**
+   * @return the the substring before the first occurrence of a separator.
+   */
+  public static String substringBefore(String str, String separator) {
+    int index = str.indexOf(separator);
+    if (index == -1) {
+      return str;
+    }
+    return str.substring(0, index);
+  }
+
+  /**
+   * @return the substring after the first occurrence of a separator.
+   */
+  public static String substringAfter(String str, String separator) {
+    int index = str.indexOf(separator);
+    if (index == -1) {
+      return EMPTY;
+    }
+    return str.substring(index + separator.length());
+  }
+}