Issue 3155. Better error report for directives in sourced unit

compiler/java/com/google/dart/compiler/resolver/Elements.java

Index: compiler/java/com/google/dart/compiler/resolver/Elements.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/Elements.java b/compiler/java/com/google/dart/compiler/resolver/Elements.java
index d0a14e99ee283865bcbfc257a816e5a8e168eec5..8a10ccb365c0853c7b0373efed2e32205d1abafb 100644
--- a/compiler/java/com/google/dart/compiler/resolver/Elements.java
+++ b/compiler/java/com/google/dart/compiler/resolver/Elements.java
@@ -499,6 +499,13 @@ static FieldElementImplementation fieldFromNode(DartField node,
   private static String getRelativeSourcePath(SourceInfo source, SourceInfo target) {
     Source sourceSource = source.getSource();
     Source targetSource = target.getSource();
+    return getRelativeSourcePath(sourceSource, targetSource);
+  }
+
+  /**
+   * @return the relative or absolute path from "source" to "target".
+   */
+  public static String getRelativeSourcePath(Source sourceSource, Source targetSource) {
     // If both source are from file, prepare relative path.
     if (sourceSource != null && targetSource != null) {
       URI sourceUri = sourceSource.getUri();