move ReplacementRange into dart specific contributor

pkg/analysis_server/lib/src/services/completion/completion_core.dart

Index: pkg/analysis_server/lib/src/services/completion/completion_core.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/completion_core.dart b/pkg/analysis_server/lib/src/services/completion/completion_core.dart
index b65f4bf7c99550f1b21eff1c8b93667f6b0d4562..7048a6e99891650a582d61ae82a8bb11cf750756 100644
--- a/pkg/analysis_server/lib/src/services/completion/completion_core.dart
+++ b/pkg/analysis_server/lib/src/services/completion/completion_core.dart
@@ -23,6 +23,22 @@ class CompletionRequestImpl implements CompletionRequest {
   @override
   final int offset;
 
+  /**
+   * The offset of the start of the text to be replaced.
+   * This will be different than the offset used to request the completion
+   * suggestions if there was a portion of an identifier before the original
+   * offset. In particular, the replacementOffset will be the offset of the
+   * beginning of said identifier.
+   */
+  int replacementOffset;
+
+  /**
+   * The length of the text to be replaced if the remainder of the identifier
+   * containing the cursor is to be replaced when the suggestion is applied
+   * (that is, the number of characters in the existing identifier).
+   */
+  int replacementLength;
+
   @override
   final ResourceProvider resourceProvider;
 
@@ -33,5 +49,19 @@ class CompletionRequestImpl implements CompletionRequest {
    * Initialize a newly created completion request based on the given arguments.
    */
   CompletionRequestImpl(this.context, this.resourceProvider, this.searchEngine,
-      this.source, this.offset);
+      this.source, this.offset) {
+    replacementOffset = offset;
+    replacementLength = 0;
+  }
+
+  /**
+   * Return the original text from the [replacementOffset] to the [offset]
+   * that can be used to filter the suggestions on the server side.
+   */
+  String get filterText {
+    return context
+        .getContents(source)
+        .data
+        .substring(replacementOffset, offset);
+  }
 }