fix(build): Restore erroneously-deleted filter function

This was deleted in PR google#7406 as it was mainly being used to
filter core/ vs. test/mocha/ deps into separate deps files -
but it turns out also to be used for filtering error
messages too.  Oops.

scripts/gulpfiles/build_tasks.js

@@ -316,6 +316,21 @@ function buildDeps() {
  /** Maximum buffer size, in bytes for child process stdout/stderr. */
  const MAX_BUFFER_SIZE = 10 * 1024 * 1024;
 
+ /**
+ * Filter a string to extract lines containing (or not containing) the
+ * specified target string.
+ *
+ * @param {string} text Text to filter.
+ * @param {string} target String to search for.
+ * @param {boolean?} exclude If true, extract only non-matching lines.
+ * @returns {string} Filtered text.
+ */
+ function filter(text, target, exclude) {
+ return text.split('\n')
+ .filter((line) => Boolean(line.match(target)) !== Boolean(exclude))
+ .join('\n');
+ }
+
  /**
  * Log unexpected diagnostics, after removing expected warnings.
  *