Use ansi color in the dartanalyzer cli output.

pkg/analyzer/lib/src/dart/analysis/driver.dart

Index: pkg/analyzer/lib/src/dart/analysis/driver.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/driver.dart b/pkg/analyzer/lib/src/dart/analysis/driver.dart
index 26ed245f22cb5f00d50d1112de2ddc519b4c48ec..5d0e00979c546fda547e9913babd6e05e598a1c7 100644
--- a/pkg/analyzer/lib/src/dart/analysis/driver.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/driver.dart
@@ -1104,8 +1104,13 @@ class AnalysisDriver implements AnalysisDriverGeneric {
         // errors were written.
         throw new StateError('No ErrorCode for $errorName in $file');
       }
-      return new AnalysisError.forValues(file.source, error.offset,
-          error.length, errorCode, error.message, error.correction);
+      return new AnalysisError.forValues(
+          file.source,
+          error.offset,
+          error.length,
+          errorCode,
+          error.message,
+          error.correction.isEmpty ? null : error.correction);
     }).toList();
   }