Add presubmit check for engine to flutter roll (#5790)

This prevents us from getting flutter far behind engine in the future.
The additional travis check will also stop the skia autoroller even
if we foget to turn it off.

.travis.yml

@@ -5,6 +5,7 @@ sudo: false
 before_script:
   - ./travis/setup.sh
 script:
+  - ./travis/check_roll.sh
   - ./travis/format.sh
   - ./travis/build.sh
   - ./travis/test.sh

travis/check_roll.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+
+if git remote get-url upstream >/dev/null 2>&1; then
+  UPSTREAM=upstream/master
+else
+  UPSTREAM=master
+fi;
+
+FLUTTER_VERSION="$(curl -s https://raw.githubusercontent.com/flutter/flutter/master/bin/internal/engine.version)"
+BEHIND="$(git rev-list $FLUTTER_VERSION..$UPSTREAM --oneline | wc -l)"
+MAX_BEHIND=16 # no more than 4 bisections to identify the issue
+
+if [[ $BEHIND -le $MAX_BEHIND ]]; then
+  echo "OK, the flutter/engine to flutter/flutter roll is only $BEHIND commits behind."
+else
+  echo "ERROR: The flutter/engine to flutter/flutter roll is $BEHIND commits behind!"
+  echo "       It exceeds our max allowance of $MAX_BEHIND. Unless that this commit fixes the roll,"
+  echo "       please roll engine into flutter first before merging more commits into engine."
+  exit 1
+fi
+