Fix testcases for isolated modules.

tests/cases/compiler/systemModuleAmbientDeclarations.ts

@@ -1,5 +1,5 @@
 // @module: system
-// @separateCompilation: true
+// @isolatedModules: true
 
 // @filename: file1.ts
 declare class Promise { }

tests/cases/compiler/systemModuleConstEnumsSeparateCompilation.ts

@@ -1,5 +1,5 @@
 // @module: system
-// @separateCompilation: true
+// @isolatedModules: true
 
 declare function use(a: any);
 const enum TopLevelConstEnum { X }

tests/cases/compiler/systemModuleDeclarationMerging.ts

@@ -1,5 +1,5 @@
 // @module: system
-// @separateCompilation: true
+// @isolatedModules: true
 
 export function F() {}
 export module F { var x; }

tests/cases/compiler/systemModuleNonTopLevelModuleMembers.ts

@@ -1,5 +1,5 @@
 // @module: system
-// @separateCompilation: true
+// @isolatedModules: true
 
 export class TopLevelClass {}
 export module TopLevelModule {var v;}