Find diagnostic locations for use in new compiler API.

dart/frog/leg/apiimpl.dart

Index: dart/frog/leg/apiimpl.dart
diff --git a/dart/frog/leg/apiimpl.dart b/dart/frog/leg/apiimpl.dart
index da73f6445a3d8d8dbfe6d2f3fb456cb64278324f..59bf84b67a85cd610bf3c797e87b2385eb9e7973 100644
--- a/dart/frog/leg/apiimpl.dart
+++ b/dart/frog/leg/apiimpl.dart
@@ -59,4 +59,12 @@ class Compiler extends leg.Compiler {
     }
     return success;
   }
+
+  void reportDiagnostic(leg.SourceSpan span, String message, bool fatal) {
+    if (span === null) {
+      handler(null, null, null, message, fatal);
+    } else {
+      handler(span.uri, span.begin, span.end, message, fatal);
+    }
+  }
 }