make importing matplotlib optional

tools/testing/browser_README.txt

 Overview:
  These are the instructions to run a wide variety of browser tests using 
  test.dart --component=webdriver or 
- dart/tools/testing/perf_testing/create_graph.py. Currently the results of
- create_graph are uploaded to https://dartperf.googleplex.com/. 
+ dart/tools/testing/perf_testing/run_perf_tests.py. Currently the results of
+ run_perf_tests are uploaded to https://dartperf.googleplex.com/. 
  
 ============== Windows Setup ==============
 
 Open Internet Explorer. Open the Internet Options dialog, and move to the
 "Security" tab. Click through Internet, Local Intranet, Trusted sites, and
 Restricted site and ensure the "Enable Protected Mode" is checked for all zones
 or unchecked for all zones. Then click to the "Advanced" tab, scroll down to the
 "Security" section, and check the checkbox "Allow active content to run in files
 on My Computer."
 
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 11)Install matplotlib http://matplotlib.sourceforge.net/
 
 12)Pull down benchmarks from internal repo (Google only): 
    http://chromegw.corp.google.com/viewvc/dash/trunk/internal/browserBenchmarks/README.txt?view=markup
 
 13)Create a directory in called appengine-python in third_party. Download and 
    install App Engine in the directory you just created. 
    http://code.google.com/appengine/downloads.html
 
 14) Run the tests! While standing in dart/tools/testing/perf_testing, run 
-    $> python create_graph.py --forever --verbose 
+    $> python run_perf_tests.py --forever --verbose 
     to run all the tests (browser performance, language correctness in the
     browser, command line performance, and self-hosted compile time and compiled
     code size). 
     
     You can run individual tests by adding the particular option (such as 
     --language) when running create_graph.py. Type "create_graph.py -h" for a 
     full list of the options.