Don't throw assertion when dart:lib cannot be found.

compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java

Index: compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java
diff --git a/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java b/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java
index e6c83d0ac7d38e247c92a8af57d2c21cee8e47f0..a65ddfaa78159be99777cd9a3dc37fef182ac93b 100644
--- a/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java
+++ b/compiler/java/com/google/dart/compiler/DefaultCompilerConfiguration.java
@@ -208,6 +208,11 @@ public class DefaultCompilerConfiguration implements CompilerConfiguration {
     } catch (URISyntaxException e) {
       throw new RuntimeException(e);
     }
+
+    // Verify the dart system library exists
+    if( null == this.systemLibraryManager.expandRelativeDartUri(systemUri) ) {
+      return null;
+    }
     return new UrlLibrarySource(systemUri, this.systemLibraryManager);
   }