Allow sentinel-only output files in checksOnly mode to make bazel happy.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123182655

src/com/google/javascript/jscomp/AbstractCommandLineRunner.java

@@ -35,6 +35,7 @@
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
 import com.google.javascript.jscomp.CompilerOptions.JsonStreamMode;
+import com.google.javascript.jscomp.CompilerOptions.OutputJs;
 import com.google.javascript.jscomp.CompilerOptions.TweakProcessing;
 import com.google.javascript.jscomp.deps.ClosureBundler;
 import com.google.javascript.jscomp.deps.SourceCodeEscapers;
@@ -973,6 +974,12 @@ static void writeOutput(Appendable out, Compiler compiler, String code,
       String wrapper, String codePlaceholder,
       @Nullable Function<String, String> escaper)
       throws IOException {
+    if (compiler.getOptions().outputJs == OutputJs.SENTINEL) {
+      out.append("// No JS output because the compiler was run in checks-only mode.\n");
+      return;
+    }
+    Preconditions.checkState(compiler.getOptions().outputJs == OutputJs.NORMAL);
+
     int pos = wrapper.indexOf(codePlaceholder);
     if (pos != -1) {
       String prefix = "";
@@ -1195,7 +1202,7 @@ int processResults(Result result, List<JSModule> modules, B options) throws IOEx
       outputBundle();
       outputModuleGraphJson();
       return 0;
-    } else if (options.outputJs && result.success) {
+    } else if (options.outputJs != OutputJs.NONE && result.success) {
       outputModuleGraphJson();
       if (modules == null) {
         outputSingleBinary(options);

src/com/google/javascript/jscomp/CommandLineRunner.java

@@ -1496,7 +1496,9 @@ protected CompilerOptions createOptions() {
     options.setEnvironment(flags.environment);
 
     options.setChecksOnly(flags.checksOnly);
-    options.setOutputJs(!flags.checksOnly);
+    if (flags.checksOnly) {
+      options.setOutputJs(CompilerOptions.OutputJs.NONE);
+    }
 
     if (flags.useTypesForOptimization) {
       level.setTypeBasedOptimizationOptions(options);

src/com/google/javascript/jscomp/CompilerOptions.java

@@ -717,9 +717,17 @@ public void setAppNameStr(String appNameStr) {
   /** Record function information */
   public boolean recordFunctionInformation;
 
-  public boolean checksOnly;
+  boolean checksOnly;
 
-  public boolean outputJs;
+  static enum OutputJs {
+    // Don't output anything.
+    NONE,
+    // Output a "sentinel" file containing just a comment.
+    SENTINEL,
+    // Output the compiled JS.
+    NORMAL,
+  }
+  OutputJs outputJs;
 
   public boolean generateExports;
 
@@ -1085,7 +1093,7 @@ public CompilerOptions() {
     appNameStr = "";
     recordFunctionInformation = false;
     checksOnly = false;
-    outputJs = true;
+    outputJs = OutputJs.NORMAL;
     generateExports = false;
     generateExportsAfterTypeChecking = true;
     exportLocalPropertyDefinitions = false;
@@ -1505,7 +1513,7 @@ public void setChecksOnly(boolean checksOnly) {
     this.checksOnly = checksOnly;
   }
 
-  public void setOutputJs(boolean outputJs) {
+  void setOutputJs(OutputJs outputJs) {
     this.outputJs = outputJs;
   }