JsChecker: support extensionless ES6 imports

Author: Rob Figueiredo

java/com/google/javascript/jscomp/CheckStrictDeps.java

@@ -136,6 +136,12 @@ private void visitEs6Import(NodeTraversal t, Node n) {
     private void checkNamespaceIsProvided(NodeTraversal t, Node n, String namespace) {
       if (namespace.startsWith("/") || namespace.startsWith(".")) {
         // TODO(jart): Unify path resolution with ModuleLoader.
+        // NOTE(robfig): To enable usage of extensionless ES6 modules,
+        // copy these 3 lines from NodeModuleResolver.java.
+        if (!namespace.endsWith(".js")) {
+          namespace += ".js";
+        }
+
         Webpath me = Webpath.get(t.getSourceName());
         if (!me.isAbsolute()) {
           me = Webpath.get("/").resolve(me);

java/com/google/javascript/jscomp/JsChecker.java

@@ -27,6 +27,7 @@
 import com.google.common.collect.Sets;
 import com.google.javascript.jscomp.CompilerOptions.IncrementalCheckMode;
 import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
+import com.google.javascript.jscomp.deps.ModuleLoader.ResolutionMode;
 import com.google.javascript.jscomp.parsing.Config;
 import io.bazel.rules.closure.BuildInfo.ClosureJsLibrary;
 import io.bazel.rules.closure.worker.CommandLineProgram;
@@ -132,6 +133,11 @@ public final class JsChecker {
       usage = "Invert exit code and disable printing warnings")
   private boolean expectFailure;
 
+  @Option(
+      name = "--module_resolution",
+      usage = "Specifies how the compiler locates modules")
+  private String moduleResolution = ResolutionMode.BROWSER.name();
+
   @Option(
       name = "--help",
       usage = "Displays this message on stdout and exit")
@@ -182,6 +188,7 @@ private boolean run() throws IOException {
     options.setPreserveDetailedSourceInfo(true);
     options.setEmitUseStrict(false);
     options.setParseJsDocDocumentation(Config.JsDocParsing.INCLUDE_DESCRIPTIONS_NO_WHITESPACE);
+    options.setModuleResolutionMode(ResolutionMode.valueOf(moduleResolution));
     JsCheckerErrorFormatter errorFormatter =
         new JsCheckerErrorFormatter(compiler, state.roots, labels);
     errorFormatter.setColorize(true);