Report errors and warnings for hiding elements, issue 572.

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

Index: compiler/java/com/google/dart/compiler/util/Paths.java
diff --git a/compiler/java/com/google/dart/compiler/util/Paths.java b/compiler/java/com/google/dart/compiler/util/Paths.java
index d2a6b0de5c35d1a56431766a99136632d4748b72..ee42826f074be9bf236145d989d2f7bd2024c58e 100644
--- a/compiler/java/com/google/dart/compiler/util/Paths.java
+++ b/compiler/java/com/google/dart/compiler/util/Paths.java
@@ -30,6 +30,11 @@ public class Paths {
     int baseFilePathLen = baseFilePath.length();
     int relFilePathLen = relFilePath.length();
 
+    // Check for empty cases
+    if (baseFilePathLen == 0 && relFilePathLen == 0) {
+      return relFilePath;
+    }
+
     // Find the common path elements
     int index = 0;
     while (index < baseFilePathLen - 1 && index < relFilePathLen - 1) {