Rework "analysis.updateContent" analysis server request.

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index 9db8090d09461a0d2f4ca162382fc4d025d0a013..e55e23768613f123a1f18c3b9f147d36731651e6 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -483,8 +483,15 @@ class AnalysisServer {
         if (change.offset == null) {
           analysisContext.setContents(source, change.content);
         } else {
-          analysisContext.setChangedContents(source, change.content,
-              change.offset, change.oldLength, change.newLength);
+          // TODO(paulberry): an error should be generated if source is not
+          // currently in the content cache.
+          TimestampedData<String> oldContents = analysisContext.getContents(
+              source);
+          int offsetEnd = change.offset + change.oldLength;
+          String newContents = oldContents.data.substring(0, change.offset) +
+              change.content + oldContents.data.substring(offsetEnd);
+          analysisContext.setChangedContents(source, newContents, change.offset,
+              change.oldLength, change.content.length);
         }
         schedulePerformAnalysisOperation(analysisContext);
       }