Teach DeltaAnalyzer to recognize library prefixes.

compiler/java/com/google/dart/compiler/ast/LibraryUnit.java

Index: compiler/java/com/google/dart/compiler/ast/LibraryUnit.java
===================================================================
--- compiler/java/com/google/dart/compiler/ast/LibraryUnit.java	(revision 6284)
+++ compiler/java/com/google/dart/compiler/ast/LibraryUnit.java	(working copy)
@@ -170,6 +170,12 @@
     return new HashSet<String>(prefixes.values());
   }
 
+  public void initializePrefixes(LibraryUnit source) {
+    for (Map.Entry<LibraryUnit,String> e : source.prefixes.entrySet()) {
+      prefixes.put(e.getKey(), e.getValue());
+    }
+  }
+
   /**
    * Return the path for dart source that corresponds to the same dart file as
    * this library unit. This is added to the set of sourcePaths for this unit.