Demote "less important" library-level checking diagnostics to warnings

This change adds a warnings guard to suppress diagnostics inside .i.js files,
which generally correspond to issues in transitive dependencies. This feature
can be used during library-level checking to make sure that only issues found
inside the library being checked are promoted to (build-breaking) errors, while
issues inside transitive dependencies are (non-breaking) warnings instead.

The intention is that using this guard should give the most useful diagnostics
earlier during library-level checking.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181245720

src/com/google/javascript/jscomp/ijs/CheckIjsWarningsGuard.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2018 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.javascript.jscomp.ijs;
+
+import com.google.javascript.jscomp.CheckLevel;
+import com.google.javascript.jscomp.JSError;
+import com.google.javascript.jscomp.WarningsGuard;
+
+/**
+ * A warnings guard that sets the errors found in .i.js files to be warnings, and only the warnings
+ * found in the library in question to errors.
+ */
+public class CheckIjsWarningsGuard extends WarningsGuard {
+
+  @Override
+  public CheckLevel level(JSError error) {
+    if (error.sourceName.endsWith(".i.js")) {
+      return CheckLevel.WARNING;
+    }
+    return null;
+  }
+}