Allow specmap to have methods to branch into multiple sb3 opcodes

src/serialization/sb2.js

@@ -447,6 +447,24 @@ const sb2import = function (json, runtime, optForceSprite) {
         }));
 };
 
+/**
+ * Given the sb2 block, inspect the specmap for a translation method or object.
+ * @param {!object} block a sb2 formatted block
+ * @return {object} specmap block to parse this opcode
+ */
+const specMapBlock = function (block) {
+    const opcode = block[0];
+    const mapped = opcode && specMap[opcode];
+    if (!mapped) {
+        log.warn(`Couldn't find SB2 block: ${opcode}`);
+        return null;
+    }
+    if (typeof mapped === 'function') {
+        return mapped(block);
+    }
+    return mapped;
+};
+
 /**
  * Parse a single SB2 JSON-formatted block and its children.
  * @param {!object} sb2block SB2 JSON-formatted block.
@@ -456,14 +474,11 @@ const sb2import = function (json, runtime, optForceSprite) {
  * @return {object} Scratch VM format block, or null if unsupported object.
  */
 const parseBlock = function (sb2block, addBroadcastMsg, getVariableId, extensions) {
-    // First item in block object is the old opcode (e.g., 'forward:').
-    const oldOpcode = sb2block[0];
-    // Convert the block using the specMap. See sb2specmap.js.
-    if (!oldOpcode || !specMap[oldOpcode]) {
-        log.warn('Couldn\'t find SB2 block: ', oldOpcode);
-        return null;
+    const blockMetadata = specMapBlock(sb2block);
+    if (!blockMetadata) {
+        return;
     }
-    const blockMetadata = specMap[oldOpcode];
+    const oldOpcode = sb2block[0];
     // If the block is from an extension, record it.
     const dotIndex = blockMetadata.opcode.indexOf('.');
     if (dotIndex >= 0) {

src/serialization/sb2_specmap.js

@@ -652,7 +652,22 @@ const specMap = {
             }
         ]
     },
-    'whenSensorGreaterThan': {
+    'whenSensorGreaterThan': ([, sensor]) => {
+        if (sensor === 'video motion') {
+            return {
+                opcode: 'videoSensing.whenMotionGreaterThan',
+                argMap: [
+                    // skip the first arg, since we converted to a video specific sensing block
+                    {},
+                    {
+                        type: 'input',
+                        inputOp: 'math_number',
+                        inputName: 'REFERENCE'
+                    }
+                ]
+            };
+        }
+        return {
             opcode: 'event_whengreaterthan',
             argMap: [
                 {
@@ -665,6 +680,7 @@ const specMap = {
                     inputName: 'VALUE'
                 }
             ]
+        };
     },
     'whenIReceive': {
         opcode: 'event_whenbroadcastreceived',