Rework recent changes to LineInfo to be backward compatible.

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

Index: pkg/analyzer/lib/src/generated/source.dart
diff --git a/pkg/analyzer/lib/src/generated/source.dart b/pkg/analyzer/lib/src/generated/source.dart
index 8d5d3fde643866a2d7d5c6141faf7ce670766b64..0fae83a01853d234957ecf8969010ff3314a0e34 100644
--- a/pkg/analyzer/lib/src/generated/source.dart
+++ b/pkg/analyzer/lib/src/generated/source.dart
@@ -197,7 +197,13 @@ class LineInfo {
    * Initialize a newly created set of line information to represent the data
    * encoded in the given list of [_lineStarts].
    */
-  LineInfo(this._lineStarts) {
+  factory LineInfo(List<int> _lineStarts) => new LineInfoWithCount(_lineStarts);
+
+  /**
+   * Initialize a newly created set of line information to represent the data
+   * encoded in the given list of [_lineStarts].
+   */
+  LineInfo._(this._lineStarts) {
     if (_lineStarts == null) {
       throw new IllegalArgumentException("lineStarts must be non-null");
     } else if (_lineStarts.length < 1) {
@@ -206,11 +212,6 @@ class LineInfo {
   }
 
   /**
-   * Return the number of lines in the file.
-   */
-  int get lineCount => _lineStarts.length;
-
-  /**
    * Return the location information for the character at the given [offset].
    */
   LineInfo_Location getLocation(int offset) {
@@ -288,6 +289,26 @@ class LineInfo_Location {
 }
 
 /**
+ * Information about line and column information within a source file,
+ * including a count of the total number of lines.
+ *
+ * TODO(paulberry): in the next major version roll of analyzer, merge this
+ * class into [LineInfo].
+ */
+class LineInfoWithCount extends LineInfo {
+  /**
+   * Initialize a newly created set of line information to represent the data
+   * encoded in the given list of [_lineStarts].
+   */
+  LineInfoWithCount(List<int> _lineStarts) : super._(_lineStarts);
+
+  /**
+   * Return the number of lines in the file.
+   */
+  int get lineCount => _lineStarts.length;
+}
+
+/**
  * Instances of interface `LocalSourcePredicate` are used to determine if the given
  * [Source] is "local" in some sense, so can be updated.
  */