refactor(compiler): simplify the CompilerHost used for transformers

- remove unneeded methods (`getNgCanonicalFileName`, `assumeFileExists`)
- simplify moduleName <-> fileName conversion logic as we don’t need to
  account for `genDir` anymore.
- rename `createNgCompilerHost` -> `createCompilerHost`

packages/compiler-cli/test/transformers/compiler_host_spec.ts

@@ -0,0 +1,98 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import * as ts from 'typescript';
+
+import {CompilerHost, CompilerOptions} from '../../src/transformers/api';
+import {createCompilerHost} from '../../src/transformers/compiler_host';
+import {Directory, Entry, MockAotContext, MockCompilerHost} from '../mocks';
+
+const dummyModule = 'export let foo: any[];';
+
+describe('NgCompilerHost', () => {
+  function createHost(
+      {files = {}, options = {basePath: '/tmp'}}: {files?: Directory,
+                                                   options?: CompilerOptions} = {}) {
+    const context = new MockAotContext('/tmp/', files);
+    const tsHost = new MockCompilerHost(context);
+    return createCompilerHost({tsHost, options});
+  }
+
+  describe('fileNameToModuleName', () => {
+    let ngHost: CompilerHost;
+    beforeEach(() => { ngHost = createHost(); });
+
+    it('should use a package import when accessing a package from a source file', () => {
+      expect(ngHost.fileNameToModuleName('/tmp/node_modules/@angular/core.d.ts', '/tmp/main.ts'))
+          .toBe('@angular/core');
+    });
+
+    it('should use a package import when accessing a package from another package', () => {
+      expect(ngHost.fileNameToModuleName(
+                 '/tmp/node_modules/mod1/index.d.ts', '/tmp/node_modules/mod2/index.d.ts'))
+          .toBe('mod1/index');
+      expect(ngHost.fileNameToModuleName(
+                 '/tmp/node_modules/@angular/core/index.d.ts',
+                 '/tmp/node_modules/@angular/common/index.d.ts'))
+          .toBe('@angular/core/index');
+    });
+
+    it('should use a relative import when accessing a file in the same package', () => {
+      expect(ngHost.fileNameToModuleName(
+                 '/tmp/node_modules/mod/a/child.d.ts', '/tmp/node_modules/mod/index.d.ts'))
+          .toBe('./a/child');
+      expect(ngHost.fileNameToModuleName(
+                 '/tmp/node_modules/@angular/core/src/core.d.ts',
+                 '/tmp/node_modules/@angular/core/index.d.ts'))
+          .toBe('./src/core');
+    });
+
+    it('should use a relative import when accessing a source file from a source file', () => {
+      expect(ngHost.fileNameToModuleName('/tmp/src/a/child.ts', '/tmp/src/index.ts'))
+          .toBe('./a/child');
+    });
+
+    it('should support multiple rootDirs when accessing a source file form a source file', () => {
+      const ngHostWithMultipleRoots = createHost({
+        options: {
+          basePath: '/tmp/',
+          rootDirs: [
+            'src/a',
+            'src/b',
+          ]
+        }
+      });
+      expect(ngHostWithMultipleRoots.fileNameToModuleName('/tmp/src/b/b.ts', '/tmp/src/a/a.ts'))
+          .toBe('./b');
+    });
+
+    it('should error if accessing a source file from a package', () => {
+      expect(
+          () => ngHost.fileNameToModuleName(
+              '/tmp/src/a/child.ts', '/tmp/node_modules/@angular/core.d.ts'))
+          .toThrowError(
+              'Trying to import a source file from a node_modules package: import /tmp/src/a/child.ts from /tmp/node_modules/@angular/core.d.ts');
+    });
+
+  });
+
+  describe('moduleNameToFileName', () => {
+    it('should resolve a package import without a containing file', () => {
+      const ngHost = createHost(
+          {files: {'tmp': {'node_modules': {'@angular': {'core': {'index.d.ts': dummyModule}}}}}});
+      expect(ngHost.moduleNameToFileName('@angular/core'))
+          .toBe('/tmp/node_modules/@angular/core/index.d.ts');
+    });
+
+    it('should resolve an import using the containing file', () => {
+      const ngHost = createHost({files: {'tmp': {'src': {'a': {'child.d.ts': dummyModule}}}}});
+      expect(ngHost.moduleNameToFileName('./a/child', '/tmp/src/index.ts'))
+          .toBe('/tmp/src/a/child.d.ts');
+    });
+  });
+});