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Tasty hardening: Tasty with missing fields in classes isn't detected early enough

See original GitHub issue

I keep getting, even on master, this failure on local runs:

> testOnly dotty.tools.dotc.FromTastyTests
[info] Cleaning the dotty-sbt-bridge cache
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0
[info] No tests to run for dotty-interfaces/test:testOnly
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0
[info] No tests to run for dotty-library/test:testOnly
[warn] Multiple main classes detected.  Run 'show discoveredMainClasses' to see the list
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0
[info] No tests to run for dotty/test:testOnly
[warn] Multiple main classes detected.  Run 'show discoveredMainClasses' to see the list
[info] Test run started
[info] Test dotty.tools.dotc.FromTastyTests.runTestFromTasty started
[=======================================] completed (944/944, 67s)
[=======================================] completed (898/898, 40s)  s)
[=======================================] completed (1/1, 0s)
[info] Test dotty.tools.dotc.FromTastyTests.posTestFromTasty started
[=======================================] completed (1466/1466, 71s)
undefined: B2.t.f # 6948988: TermRef(TermRef(ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <root>)),module class <empty>)),module class B2$)),module t),f) at frontend
[=======================================] completed (1362/1362, 33s)
[error] Test dotty.tools.dotc.FromTastyTests.posTestFromTasty failed: java.lang.AssertionError: Expected no errors when compiling, failed for the following reason(s):
[error]
[error]   - encountered 1 error(s), took 104.822 sec
[error]     at dotty.tools.vulpix.ParallelTesting$CompilationTest.checkCompile(ParallelTesting.scala:958)
[error]     at dotty.tools.dotc.FromTastyTests.posTestFromTasty(FromTastyTests.scala:68)
[error]     ...

================================================================================
Test Report
================================================================================

4 passed, 1 failed, 5 total
    tests/pos/overloaded_ho_fun.scala failed


--------------------------------------------------------------------------------
Note - reproduction instructions have been dumped to log file:
    /Users/pgiarrusso/git/dotty/testlogs/tests-2018-04-05/tests-2018-04-05-T21-27-32.log
--------------------------------------------------------------------------------
[info] Test run finished: 1 failed, 0 ignored, 2 total, 214.307s
[error] Failed: Total 2, Failed 1, Errors 0, Passed 1
[error] Failed tests:
[error] 	dotty.tools.dotc.FromTastyTests
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0
[info] No tests to run for dotty-sbt-bridge/test:testOnly
[info] Passed: Total 0, Failed 0, Errors 0, Passed 0
[info] No tests to run for dotty-doc/test:testOnly
[error] (dotty-compiler/test:testOnly) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 218 s, completed Apr 5, 2018 9:31:08 PM