Improve meteor shell command evaluation.

Specific improvements:

- Parentheses are now stripped from commands that look like named classes
  so that they will be treated as class declarations rather than as named
  class expressions.

- When the `ecmascript` package is installed, `compileForShell` errors are
  now exposed to the `evalCommand` callback.

- Instead of using `vm.runInThisContext` to parse and evaluate commands at
  the same time, `evalCommand` now parses commands by creating a new
  `vm.Script` and later evaluates them using `script.runInThisContext()`,
  so that `SyntaxError`s can be reported immediately. Fixes #5131.

tools/static-assets/server/shell-server.js

@@ -266,21 +266,54 @@ var evalCommandPromise = Promise.resolve();
 
 function evalCommand(command, context, filename, callback) {
   if (Package.ecmascript) {
+    var noParens = stripParens(command);
+    if (noParens !== command) {
+      var classMatch = /^\s*class\s+(\w+)/.exec(noParens);
+      if (classMatch && classMatch[1] !== "extends") {
+        // If the command looks like a named ES2015 class, we remove the
+        // extra layer of parentheses added by the REPL so that the
+        // command will be evaluated as a class declaration rather than as
+        // a named class expression. Note that you can still type (class A
+        // {}) explicitly to evaluate a named class expression. The REPL
+        // code that calls evalCommand handles named function expressions
+        // similarly (first with and then without parentheses), but that
+        // code doesn't know about ES2015 classes, which is why we have to
+        // handle them here.
+        command = noParens;
+      }
+    }
+
     try {
-      command = command.replace(/^\(|\)$/g, "");
       command = Package.ecmascript.ECMAScript.compileForShell(command);
     } catch (error) {
-      // If there was an error compiling the original command, there's a
-      // chance it didn't need to be compiled at all, so we might as well
-      // let vm.runInThisContext try to evaluate it.
+      callback(error);
+      return;
     }
   }
 
+  try {
+    var script = new vm.Script(command, {
+      filename: filename,
+      displayErrors: false
+    });
+  } catch (parseError) {
+    callback(parseError);
+    return;
+  }
+
   evalCommandPromise.then(function () {
-    callback(null, vm.runInThisContext(command, filename));
+    callback(null, script.runInThisContext());
   }).catch(callback);
 }
 
+function stripParens(command) {
+  if (command.charAt(0) === "(" &&
+      command.charAt(command.length - 1) === ")") {
+    return command.slice(1, command.length - 1);
+  }
+  return command;
+}
+
 // This function allows a persistent history of shell commands to be saved
 // to and loaded from .meteor/local/shell-history.
 Sp.initializeHistory = function initializeHistory() {