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

compiler/javatests/com/google/dart/compiler/MockLibrarySource.java

Index: compiler/javatests/com/google/dart/compiler/MockLibrarySource.java
diff --git a/compiler/javatests/com/google/dart/compiler/MockLibrarySource.java b/compiler/javatests/com/google/dart/compiler/MockLibrarySource.java
index d5696cb1840aec97f8a0ddf345eddd2df641ef1a..7c06ffbd4072fa5a4e119fd98d2d7326c3c63e1e 100644
--- a/compiler/javatests/com/google/dart/compiler/MockLibrarySource.java
+++ b/compiler/javatests/com/google/dart/compiler/MockLibrarySource.java
@@ -46,7 +46,7 @@ public class MockLibrarySource extends SourceTest implements LibrarySource {
         return lib;
       }
     }
-    throw new RuntimeException("Cannot find import for " + relPath);
+    return null;
   }
 
   @Override
@@ -59,7 +59,7 @@ public class MockLibrarySource extends SourceTest implements LibrarySource {
         return source;
       }
     }
-    throw new RuntimeException("Cannot find source for " + relPath);
+    return null;
   }
 
   public void addSource(DartSource src) {