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.
    *