Add Thread.blockContainer

Store the thread's blocks at blockContainer letting execute quickly
determine the block source. Monitor threads are a monitor thread. They
do not become a target thread suddenly.

src/engine/execute.js

@@ -126,12 +126,7 @@ const execute = function (sequencer, thread, recursiveCall) {
     const currentBlockId = thread.peekStack();
     const currentStackFrame = thread.peekStackFrame();
 
-    let blockContainer;
+    let blockContainer = thread.blockContainer;
-    if (thread.updateMonitor) {
-        blockContainer = runtime.monitorBlocks;
-    } else {
-        blockContainer = target.blocks;
-    }
     let block = blockContainer.getBlock(currentBlockId);
     if (typeof block === 'undefined') {
         blockContainer = runtime.flyoutBlocks;

src/engine/runtime.js

@@ -861,6 +861,9 @@ class Runtime extends EventEmitter {
         thread.target = target;
         thread.stackClick = opts.stackClick;
         thread.updateMonitor = opts.updateMonitor;
+        thread.blockContainer = opts.updateMonitor ?
+            this.monitorBlocks :
+            target.blocks;
 
         thread.pushStack(id);
         this.threads.push(thread);
@@ -890,6 +893,7 @@ class Runtime extends EventEmitter {
         newThread.target = thread.target;
         newThread.stackClick = thread.stackClick;
         newThread.updateMonitor = thread.updateMonitor;
+        newThread.blockContainer = thread.blockContainer;
         newThread.pushStack(thread.topBlock);
         const i = this.threads.indexOf(thread);
         if (i > -1) {

src/engine/thread.js

@@ -42,6 +42,12 @@ class Thread {
          */
         this.target = null;
 
+        /**
+         * The Blocks this thread will execute.
+         * @type {Blocks}
+         */
+        this.blockContainer = null;
+
         /**
          * Whether the thread requests its script to glow during this frame.
          * @type {boolean}

test/unit/engine_sequencer.js

@@ -91,7 +91,8 @@ const generateThread = function (runtime) {
     rt.blocks.createBlock(generateBlock(next));
     th.pushStack(next);
     th.target = rt;
-    
+    th.blockContainer = rt.blocks;
+
     runtime.threads.push(th);
 
     return th;