Replace and remove JavaException(s).

pkg/analyzer/lib/src/context/source.dart

Index: pkg/analyzer/lib/src/context/source.dart
diff --git a/pkg/analyzer/lib/src/context/source.dart b/pkg/analyzer/lib/src/context/source.dart
index 0c9a84fd7e46a4f12cf0aa1d6b3023851818c4ce..d6f5c15ca14e8699e054a9232b646ad4badc9dcf 100644
--- a/pkg/analyzer/lib/src/context/source.dart
+++ b/pkg/analyzer/lib/src/context/source.dart
@@ -143,8 +143,7 @@ class SourceFactoryImpl implements SourceFactory {
   Source fromEncoding(String encoding) {
     Source source = forUri(encoding);
     if (source == null) {
-      throw new IllegalArgumentException(
-          "Invalid source encoding: '$encoding'");
+      throw new ArgumentError("Invalid source encoding: '$encoding'");
     }
     return source;
   }