Add a utility method so that it can be added to the superclass without breaking the build

lib/src/linter.dart

Index: lib/src/linter.dart
diff --git a/lib/src/linter.dart b/lib/src/linter.dart
index a7c0a57d2f33d129e3773e1084b1fcae083d0d82..f58b81dba1944b770e0e79591e4acb272148d76c 100644
--- a/lib/src/linter.dart
+++ b/lib/src/linter.dart
@@ -425,6 +425,10 @@ class _LineInfo implements LineInfo {
   @override
   LineInfo_Location getLocation(int offset) => new LineInfo_Location(
       node.span.start.line + 1, node.span.start.column + 1);
+
+  int getOffsetOfLine(int lineNumber) {
+    throw new UnsupportedError('Cannot get line offset from a yaml node');
+  }
 }
 
 class _LintCode extends LintCode {