Add missing type parameter in TestDiagnosticMessages.

This was cause of an unchecked cast warning during build.

Change-Id: I02bd1caf8dac9b72692bda6893eacb19cc819a5f
diff --git a/src/test/java/com/android/tools/r8/TestDiagnosticMessages.java b/src/test/java/com/android/tools/r8/TestDiagnosticMessages.java
index ac3ef82..ed0e8f1 100644
--- a/src/test/java/com/android/tools/r8/TestDiagnosticMessages.java
+++ b/src/test/java/com/android/tools/r8/TestDiagnosticMessages.java
@@ -36,7 +36,7 @@
 
   // Match exact.
 
-  default TestDiagnosticMessages assertDiagnosticsMatch(Matcher matcher) {
+  default TestDiagnosticMessages assertDiagnosticsMatch(Matcher<Diagnostic> matcher) {
     return assertDiagnosticsMatch(Collections.singletonList(matcher));
   }