If a commonjs module import cannot be located, provide a fallback

Provide a fallback mechanism so we don't leave a stray `require()` call.
Useful for whitespace_only mode and per-file rewriting.

Closes #2695

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176438020

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

@@ -401,27 +401,22 @@ public void visit(NodeTraversal t, Node n, Node parent) {
 
     /** Visit require calls. */
     private void visitRequireCall(NodeTraversal t, Node require, Node parent) {
-      String requireName = ProcessCommonJSModules.getCommonJsImportPath(require);
-      ModulePath modulePath =
-          t.getInput()
-              .getPath()
-              .resolveJsModule(
-                  requireName,
-                  require.getSourceFileName(),
-                  require.getLineno(),
-                  require.getCharno());
-      if (modulePath == null) {
-        // The module loader will issue an error
-        return;
-      }
-
       // When require("name") is used as a standalone statement (the result isn't used)
       // it indicates that a module is being loaded for the side effects it produces.
       // In this case the require statement should just be removed as the dependency
       // sorting will insert the file for us.
       if (!NodeUtil.isExpressionResultUsed(require)
           && parent.isExprResult()
           && NodeUtil.isStatementBlock(parent.getParent())) {
+
+        // Attempt to resolve the module so that load warnings are issued
+        t.getInput()
+            .getPath()
+            .resolveJsModule(
+                getCommonJsImportPath(require),
+                require.getSourceFileName(),
+                require.getLineno(),
+                require.getCharno());
         Node grandparent = parent.getParent();
         parent.detach();
         compiler.reportChangeToEnclosingScope(grandparent);
@@ -912,12 +907,14 @@ private void visitRequireCall(NodeTraversal t, Node require, Node parent) {
                   require.getSourceFileName(),
                   require.getLineno(),
                   require.getCharno());
+
+      String moduleName;
       if (modulePath == null) {
-        // The module loader will issue an error
-        return;
+        // The module loader will issue an error, but use a fallback
+        moduleName = ModuleIdentifier.forFile(requireName).getModuleName();
+      } else {
+        moduleName = modulePath.toModuleName();
       }
-
-      String moduleName = modulePath.toModuleName();
       Node moduleRef = IR.name(moduleName).srcref(require);
       parent.replaceChild(require, moduleRef);
 
@@ -1436,10 +1433,13 @@ private String getModuleImportName(NodeTraversal t, Node n) {
                   .getPath()
                   .resolveJsModule(
                       requireName, n.getSourceFileName(), n.getLineno(), n.getCharno());
+          String moduleName;
           if (modulePath == null) {
-            return null;
+            moduleName = ModuleIdentifier.forFile(requireName).getModuleName();
+          } else {
+            moduleName = modulePath.toModuleName();
           }
-          return modulePath.toModuleName() + propSuffix;
+          return moduleName + propSuffix;
         }
         return null;
 
@@ -1480,12 +1480,15 @@ private void fixTypeNode(NodeTraversal t, Node typeNode) {
                       typeNode.getSourceFileName(),
                       typeNode.getLineno(),
                       typeNode.getCharno());
+
+          String globalModuleName;
           if (modulePath == null) {
-            // The module loader will issue an error
-            return;
+            // The module loader will issue an error, but we fall back to a path-based name
+            globalModuleName = ModuleIdentifier.forFile(moduleName).getModuleName();
+          } else {
+            globalModuleName = modulePath.toModuleName();
           }
 
-          String globalModuleName = modulePath.toModuleName();
           typeNode.setString(
               localTypeName == null ? globalModuleName : globalModuleName + localTypeName);
 

src/com/google/javascript/jscomp/parsing/ParserConfig.properties

@@ -178,6 +178,7 @@ jsdoc.suppressions =\
     missingProvide,\
     missingRequire,\
     missingReturn,\
+    moduleLoad,\
     newCheckTypes,\
     newCheckTypesAllChecks,\
     nonStandardJsDocs,\

test/com/google/javascript/jscomp/ProcessCommonJSModulesTest.java

@@ -941,5 +941,20 @@ public void testIssue2616() {
 
   public void testMissingRequire() {
     ModulesTestUtils.testModulesError(this, "require('missing');", ModuleLoader.LOAD_WARNING);
+
+    testModules(
+        "test.js",
+        lines(
+            "/**",
+            " * @fileoverview",
+            " * @suppress {moduleLoad}",
+            " */",
+            "var foo = require('missing');"),
+        lines(
+            "/**",
+            " * @fileoverview",
+            " * @suppress {moduleLoad}",
+            " */",
+            "var foo = module$missing;"));
   }
 }