Handle NPE case for 'dart:typo'

compiler/java/com/google/dart/compiler/UrlSource.java

Index: compiler/java/com/google/dart/compiler/UrlSource.java
diff --git a/compiler/java/com/google/dart/compiler/UrlSource.java b/compiler/java/com/google/dart/compiler/UrlSource.java
index 67987f4f10cb60b513b745a5d75d855ef5ee0cf6..6411b0512bd4e9e962d7efe5c23f947d6b07986e 100644
--- a/compiler/java/com/google/dart/compiler/UrlSource.java
+++ b/compiler/java/com/google/dart/compiler/UrlSource.java
@@ -49,6 +49,10 @@ public abstract class UrlSource implements Source {
 
   protected UrlSource(URI uri, SystemLibraryManager slm) {
     URI expanded = slm != null ? slm.expandRelativeDartUri(uri) : uri;
+    if (expanded == null) {
+      // import("dart:typo") case
+      expanded = uri;
+    }
     this.uri = BASE_URI.relativize(expanded.normalize());
     this.absoluteUri = BASE_URI.resolve(expanded);
     this.systemLibraryManager = slm;