ISSUE-2: VMEmulator built-in classes not useable from command line

The use of built-in classes is now controlled by the environment
variable N2T_VM_USE_BUILTINS.

If set to "yes" (case insesitive), the built-in classes will be
used without prompting.

If set to "no", the built-in classes won't be used (again, no
prompting).

In any other case, we'll fallback to the previous behaviour. That
is, prompt for GUI and reject for CLI.

Author: itoshkov

SimulatorsPackage/src/main/java/Hack/VMEmulator/CPU.java

@@ -108,10 +108,9 @@ public CPU(VMProgram program, RAM ram, CallStack callStack,
 
         stackFrames = new Vector<Integer>();
 
-        if (program.getGUI() != null) {
-            builtInFunctionsRunner =
-                new BuiltInFunctionsRunner(this, builtInDir);
-        }
+        final boolean useBuiltIns = "yes".equalsIgnoreCase(System.getenv("N2T_VM_USE_BUILTINS"));
+        if (useBuiltIns || program.getGUI() != null)
+            builtInFunctionsRunner = new BuiltInFunctionsRunner(this, builtInDir);
     }
 
     /**

SimulatorsPackage/src/main/java/Hack/VMEmulator/VMProgram.java

@@ -557,12 +557,11 @@ public short getAddress(String functionName) throws ProgramException {
                 // this as this is not a feature from the book but a later
                 // addition.
 				if (builtInAccessStatus == BUILTIN_ACCESS_UNDECIDED) {
-					if (hasGUI && gui.confirmBuiltInAccess()) {
-						builtInAccessStatus = BUILTIN_ACCESS_AUTHORIZED;
-					} else {
-						builtInAccessStatus = BUILTIN_ACCESS_DENIED;
-					}
-				}
+                    final String envUseBuiltIns = System.getenv("N2T_VM_USE_BUILTINS");
+                    final boolean useBuiltIns = "yes".equalsIgnoreCase(envUseBuiltIns)
+                            || (!"no".equalsIgnoreCase(envUseBuiltIns) && hasGUI && gui.confirmBuiltInAccess());
+                    builtInAccessStatus = useBuiltIns ? BUILTIN_ACCESS_AUTHORIZED : BUILTIN_ACCESS_DENIED;
+                }
 				if (builtInAccessStatus == BUILTIN_ACCESS_AUTHORIZED) {
 					return BUILTIN_FUNCTION_ADDRESS;
 				}