Update [getErrorMessage] to avoid throwing away useful information.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 0f59bf93d350a443ffc7ae21021ed835bab0d652..36790001ab30116c63e2acdb03d15ff79e3a4497 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -316,22 +316,16 @@ class CrossIsolateException implements Exception {
   String toString() => "\$message\\n\$stackTrace";
 }
 
-// Get a string description of an exception.
-//
-// Most exception types have a "message" property. We prefer this since
-// it skips the "Exception:", "HttpException:", etc. prefix that calling
-// toString() adds. But, alas, "message" isn't actually defined in the
-// base Exception type so there's no easy way to know if it's available
-// short of a giant pile of type tests for each known exception type.
-//
-// So just try it. If it throws, default to toString().
-String getErrorMessage(error) {
-  try {
-    return error.message;
-  } on NoSuchMethodError catch (_) {
-    return error.toString();
-  }
-}
+/// A regular expression to match the exception prefix that some exceptions'
+/// [Object.toString] values contain.
+final _exceptionPrefix = new RegExp(r'^([A-Z][a-zA-Z]*)?(Exception|Error): ');
+
+/// Get a string description of an exception.
+///
+/// Many exceptions include the exception class name at the beginning of their
+/// [toString], so we remove that if it exists.
+String getErrorMessage(error) =>
+  error.toString().replaceFirst(_exceptionPrefix, '');
 
 /// Returns a buffered stream that will emit the same values as the stream
 /// returned by [future] once [future] completes. If [future] completes to an