Removed Type-Defect label.
Added Type-Enhancement label.

logical_expression_test.dart is still failing.

The mystery 'not' comes from here:
During CFG constructions a || b is is converted into CFG blocks equivalent to !a ? b : true.

In CFG codegen, diamond control flow for x ? y : true is generated as the expression !x || y (provided x and y can be generated as expressions).
If the HNot is generated-at-use, the two negations cancel out in generateNot

When the HNot is a common subexpression the 'nots' no longer cancel out, leading to the observed strange code.

How to fix this?

Converting a || b to a ? true : b and later recognizing x ? true : y as x || y would avoid the negations.
However, the code is worse when y cannot be emitted as an expression, since then it becomes an if statement where the code is in the else-part. Perhaps that can be fixed by adding the negation only when and if statement is needed (if (e) ; else S; --> if (!e) S;).
dart2js currently gets confused for nested && and || patterns. Perhaps using a ? true : b would help tell the pattern for || apart from that for && (which is a ? b : false, with the constant then on the other arm). Or perhaps, with more conditional control flow available in modern JavaScript (??, ??=, a?.b), dart2js should remember which control-flow operator matches the CFG.