Issue 1548. If URI can not be parsed, return null.

compiler/java/com/google/dart/compiler/DeltaAnalyzer.java

Index: compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
diff --git a/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java b/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
index 60d8eee79ab09d6b730d5a611e39018c4f5f2feb..2c6161126e4244b249431892bf7054dd5597f232 100644
--- a/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
+++ b/compiler/java/com/google/dart/compiler/DeltaAnalyzer.java
@@ -123,12 +123,12 @@ class DeltaAnalyzer {
 
       @Override
       public DartSource getSourceFor(String relPath) {
-        throw new AssertionError();
+        return null;
       }
 
       @Override
       public LibrarySource getImportFor(String relPath) {
-        throw new AssertionError();
+        return null;
       }
     };
   }