Upgrade ASM to 6.2.1

[Godin]

As usual classes of JDK 12 EA already compiled into new bytecode version 56, so that ModifiedSystemClassRuntime can't be used without downgrade-upgrade workaround or ASM upgrade.

To notice changes in javac 12 EA unrelated to bytecode version such as #669 or even regressions such as JDK-8180141, one currently need to exclude integration tests which use agent:

$ java -version
openjdk version "12-ea" 2019-03-19
OpenJDK Runtime Environment 19.3 (build 12-ea+5)
OpenJDK 64-Bit Server VM 19.3 (build 12-ea+5, mixed mode)

$ mvn -V -B -e clean verify -Dbytecode.version=11 \
    -Djacoco.skip \
    -pl \!org.jacoco.ant.test,\!jacoco-maven-plugin.test,\!org.jacoco.examples.test,\!org.jacoco.doc,\!jacoco

Fortunately ASM 6.2.1 among other bug fixes and small improvements ( https://asm.ow2.io/versions.html , https://gitlab.ow2.org/asm/asm/compare/ASM_6_2...ASM_6_2_1 ) contains https://gitlab.ow2.org/asm/asm/merge_requests/187 with following interesting comments:

Eric Bruneton: Are there new bytecode features in this version? [...]
Remi Forax: None i'm aware of, plan for 12 are only languages changes (at that point). [...]

So to test compiler one can simply do

$ mvn clean verify -Dbytecode.version=11 \
    -Dasm.version=6.2.1

I propose to upgrade ASM anyway and maybe later think about experimental support for Java 12 class files.